diff --git a/eland/actions.py b/eland/actions.py
index 92cf4e0..ee1ceac 100644
--- a/eland/actions.py
+++ b/eland/actions.py
@@ -13,15 +13,16 @@
 # limitations under the License.
 
 from abc import ABC, abstractmethod
-
-# -------------------------------------------------------------------------------------------------------------------- #
-# PostProcessingActions                                                                                                #
-# -------------------------------------------------------------------------------------------------------------------- #
+from typing import TYPE_CHECKING
 from eland import SortOrder
 
 
+if TYPE_CHECKING:
+    import pandas as pd  # type: ignore
+
+
 class PostProcessingAction(ABC):
-    def __init__(self, action_type):
+    def __init__(self, action_type: str) -> None:
         """
         Abstract class for postprocessing actions
 
@@ -33,76 +34,74 @@ class PostProcessingAction(ABC):
         self._action_type = action_type
 
     @property
-    def type(self):
+    def type(self) -> str:
         return self._action_type
 
     @abstractmethod
-    def resolve_action(self, df):
+    def resolve_action(self, df: "pd.DataFrame") -> "pd.DataFrame":
         pass
 
     @abstractmethod
-    def __repr__(self):
+    def __repr__(self) -> str:
         pass
 
 
 class SortIndexAction(PostProcessingAction):
-    def __init__(self):
+    def __init__(self) -> None:
         super().__init__("sort_index")
 
-    def resolve_action(self, df):
+    def resolve_action(self, df: "pd.DataFrame") -> "pd.DataFrame":
         return df.sort_index()
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"('{self.type}')"
 
 
 class HeadAction(PostProcessingAction):
-    def __init__(self, count):
+    def __init__(self, count: int) -> None:
         super().__init__("head")
 
         self._count = count
 
-    def resolve_action(self, df):
+    def resolve_action(self, df: "pd.DataFrame") -> "pd.DataFrame":
         return df.head(self._count)
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"('{self.type}': ('count': {self._count}))"
 
 
 class TailAction(PostProcessingAction):
-    def __init__(self, count):
+    def __init__(self, count: int) -> None:
         super().__init__("tail")
 
         self._count = count
 
-    def resolve_action(self, df):
+    def resolve_action(self, df: "pd.DataFrame") -> "pd.DataFrame":
         return df.tail(self._count)
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"('{self.type}': ('count': {self._count}))"
 
 
 class SortFieldAction(PostProcessingAction):
-    def __init__(self, sort_params_string):
+    def __init__(self, sort_params_string: str) -> None:
         super().__init__("sort_field")
 
         if sort_params_string is None:
             raise ValueError("Expected valid string")
 
         # Split string
-        sort_params = sort_params_string.split(":")
-        if len(sort_params) != 2:
+        sort_field, _, sort_order = sort_params_string.partition(":")
+        if not sort_field or sort_order not in ("asc", "desc"):
             raise ValueError(
                 f"Expected ES sort params string (e.g. _doc:desc). Got '{sort_params_string}'"
             )
 
-        self._sort_field = sort_params[0]
-        self._sort_order = SortOrder.from_string(sort_params[1])
+        self._sort_field = sort_field
+        self._sort_order = SortOrder.from_string(sort_order)
 
-    def resolve_action(self, df):
-        if self._sort_order == SortOrder.ASC:
-            return df.sort_values(self._sort_field, True)
-        return df.sort_values(self._sort_field, False)
+    def resolve_action(self, df: "pd.DataFrame") -> "pd.DataFrame":
+        return df.sort_values(self._sort_field, self._sort_order == SortOrder.ASC)
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"('{self.type}': ('sort_field': '{self._sort_field}', 'sort_order': {self._sort_order}))"
diff --git a/eland/arithmetics.py b/eland/arithmetics.py
index 2be82ce..a821107 100644
--- a/eland/arithmetics.py
+++ b/eland/arithmetics.py
@@ -14,72 +14,88 @@
 
 from abc import ABC, abstractmethod
 from io import StringIO
+from typing import Union, List, TYPE_CHECKING, Any
 
-import numpy as np
+import numpy as np  # type: ignore
+
+if TYPE_CHECKING:
+    from .query_compiler import QueryCompiler
 
 
 class ArithmeticObject(ABC):
     @property
     @abstractmethod
-    def value(self):
+    def value(self) -> str:
         pass
 
     @abstractmethod
-    def dtype(self):
+    def dtype(self) -> np.dtype:
         pass
 
     @abstractmethod
-    def resolve(self):
+    def resolve(self) -> str:
         pass
 
     @abstractmethod
-    def __repr__(self):
+    def __repr__(self) -> str:
         pass
 
 
 class ArithmeticString(ArithmeticObject):
-    def __init__(self, value):
+    def __init__(self, value: str):
         self._value = value
 
-    def resolve(self):
+    def resolve(self) -> str:
         return self.value
 
     @property
-    def dtype(self):
+    def dtype(self) -> np.dtype:
         return np.dtype(object)
 
     @property
-    def value(self):
+    def value(self) -> str:
         return f"'{self._value}'"
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return self.value
 
 
 class ArithmeticNumber(ArithmeticObject):
-    def __init__(self, value, dtype):
+    def __init__(self, value: Union[int, float], dtype: np.dtype):
         self._value = value
         self._dtype = dtype
 
-    def resolve(self):
+    def resolve(self) -> str:
         return self.value
 
     @property
-    def value(self):
+    def value(self) -> str:
         return f"{self._value}"
 
     @property
-    def dtype(self):
+    def dtype(self) -> np.dtype:
         return self._dtype
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return self.value
 
 
 class ArithmeticSeries(ArithmeticObject):
-    def __init__(self, query_compiler, display_name, dtype):
+    """Represents each item in a 'Series' by using painless scripts
+    to evaluate each document in an index as a part of a query.
+    """
+
+    def __init__(
+        self, query_compiler: "QueryCompiler", display_name: str, dtype: np.dtype
+    ):
+        # type defs
+        self._value: str
+        self._tasks: List["ArithmeticTask"]
+
         task = query_compiler.get_arithmetic_op_fields()
+
         if task is not None:
+            assert isinstance(task._arithmetic_series, ArithmeticSeries)
             self._value = task._arithmetic_series.value
             self._tasks = task._arithmetic_series._tasks.copy()
             self._dtype = dtype
@@ -98,14 +114,14 @@ class ArithmeticSeries(ArithmeticObject):
             self._dtype = dtype
 
     @property
-    def value(self):
+    def value(self) -> str:
         return self._value
 
     @property
-    def dtype(self):
+    def dtype(self) -> np.dtype:
         return self._dtype
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         buf = StringIO()
         buf.write(f"Series: {self.value} ")
         buf.write("Tasks: ")
@@ -113,7 +129,7 @@ class ArithmeticSeries(ArithmeticObject):
             buf.write(f"{task!r} ")
         return buf.getvalue()
 
-    def resolve(self):
+    def resolve(self) -> str:
         value = self._value
 
         for task in self._tasks:
@@ -148,7 +164,7 @@ class ArithmeticSeries(ArithmeticObject):
 
         return value
 
-    def arithmetic_operation(self, op_name, right):
+    def arithmetic_operation(self, op_name: str, right: Any) -> "ArithmeticSeries":
         # check if operation is supported (raises on unsupported)
         self.check_is_supported(op_name, right)
 
@@ -156,7 +172,7 @@ class ArithmeticSeries(ArithmeticObject):
         self._tasks.append(task)
         return self
 
-    def check_is_supported(self, op_name, right):
+    def check_is_supported(self, op_name: str, right: Any) -> bool:
         # supported set is
         # series.number op_name number (all ops)
         # series.string op_name string (only add)
@@ -165,22 +181,20 @@ class ArithmeticSeries(ArithmeticObject):
         # series.int op_name string (none)
         # series.float op_name string (none)
 
-        # see end of https://pandas.pydata.org/pandas-docs/stable/getting_started/basics.html?highlight=dtype for
-        # dtype heirarchy
-        if np.issubdtype(self.dtype, np.number) and np.issubdtype(
-            right.dtype, np.number
-        ):
+        # see end of https://pandas.pydata.org/pandas-docs/stable/getting_started/basics.html?highlight=dtype
+        # for dtype hierarchy
+        right_is_integer = np.issubdtype(right.dtype, np.number)
+        if np.issubdtype(self.dtype, np.number) and right_is_integer:
             # series.number op_name number (all ops)
             return True
-        elif np.issubdtype(self.dtype, np.object_) and np.issubdtype(
-            right.dtype, np.object_
-        ):
+
+        self_is_object = np.issubdtype(self.dtype, np.object_)
+        if self_is_object and np.issubdtype(right.dtype, np.object_):
             # series.string op_name string (only add)
             if op_name == "__add__" or op_name == "__radd__":
                 return True
-        elif np.issubdtype(self.dtype, np.object_) and np.issubdtype(
-            right.dtype, np.integer
-        ):
+
+        if self_is_object and right_is_integer:
             # series.string op_name int (only mul)
             if op_name == "__mul__":
                 return True
@@ -191,22 +205,22 @@ class ArithmeticSeries(ArithmeticObject):
 
 
 class ArithmeticTask:
-    def __init__(self, op_name, object):
+    def __init__(self, op_name: str, object: ArithmeticObject):
         self._op_name = op_name
 
         if not isinstance(object, ArithmeticObject):
             raise TypeError(f"Task requires ArithmeticObject not {type(object)}")
         self._object = object
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         buf = StringIO()
         buf.write(f"op_name: {self.op_name} object: {self.object!r} ")
         return buf.getvalue()
 
     @property
-    def op_name(self):
+    def op_name(self) -> str:
         return self._op_name
 
     @property
-    def object(self):
+    def object(self) -> ArithmeticObject:
         return self._object
diff --git a/eland/common.py b/eland/common.py
index fe19c09..30d1856 100644
--- a/eland/common.py
+++ b/eland/common.py
@@ -15,10 +15,10 @@
 import re
 import warnings
 from enum import Enum
-from typing import Union, List, Tuple
+from typing import Union, List, Tuple, cast, Callable, Any
 
-import pandas as pd
-from elasticsearch import Elasticsearch
+import pandas as pd  # type: ignore
+from elasticsearch import Elasticsearch  # type: ignore
 
 # Default number of rows displayed (different to pandas where ALL could be displayed)
 DEFAULT_NUM_ROWS_DISPLAYED = 60
@@ -29,8 +29,8 @@ DEFAULT_PROGRESS_REPORTING_NUM_ROWS = 10000
 DEFAULT_ES_MAX_RESULT_WINDOW = 10000  # index.max_result_window
 
 
-def docstring_parameter(*sub):
-    def dec(obj):
+def docstring_parameter(*sub: Any) -> Callable[[Any], Any]:
+    def dec(obj: Any) -> Any:
         obj.__doc__ = obj.__doc__.format(*sub)
         return obj
 
@@ -42,21 +42,21 @@ class SortOrder(Enum):
     DESC = 1
 
     @staticmethod
-    def reverse(order):
+    def reverse(order: "SortOrder") -> "SortOrder":
         if order == SortOrder.ASC:
             return SortOrder.DESC
 
         return SortOrder.ASC
 
     @staticmethod
-    def to_string(order):
+    def to_string(order: "SortOrder") -> str:
         if order == SortOrder.ASC:
             return "asc"
 
         return "desc"
 
     @staticmethod
-    def from_string(order):
+    def from_string(order: str) -> "SortOrder":
         if order == "asc":
             return SortOrder.ASC
 
@@ -276,11 +276,13 @@ def es_version(es_client: Elasticsearch) -> Tuple[int, int, int]:
     property if one doesn't exist yet for the current Elasticsearch version.
     """
     if not hasattr(es_client, "_eland_es_version"):
-        major, minor, patch = [
-            int(x)
-            for x in re.match(
-                r"^(\d+)\.(\d+)\.(\d+)", es_client.info()["version"]["number"]
-            ).groups()
-        ]
+        version_info = es_client.info()["version"]["number"]
+        match = re.match(r"^(\d+)\.(\d+)\.(\d+)", version_info)
+        if match is None:
+            raise ValueError(
+                f"Unable to determine Elasticsearch version. "
+                f"Received: {version_info}"
+            )
+        major, minor, patch = [int(x) for x in match.groups()]
         es_client._eland_es_version = (major, minor, patch)
-    return es_client._eland_es_version
+    return cast(Tuple[int, int, int], es_client._eland_es_version)
diff --git a/eland/dataframe.py b/eland/dataframe.py
index 719d1de..346619e 100644
--- a/eland/dataframe.py
+++ b/eland/dataframe.py
@@ -187,7 +187,7 @@ class DataFrame(NDFrame):
         """
         return len(self.columns) == 0 or len(self.index) == 0
 
-    def head(self, n=5):
+    def head(self, n: int = 5) -> "DataFrame":
         """
         Return the first n rows.
 
@@ -222,7 +222,7 @@ class DataFrame(NDFrame):
         """
         return DataFrame(query_compiler=self._query_compiler.head(n))
 
-    def tail(self, n=5):
+    def tail(self, n: int = 5) -> "DataFrame":
         """
         Return the last n rows.
 
diff --git a/eland/filter.py b/eland/filter.py
index cf0387b..fe1de3d 100644
--- a/eland/filter.py
+++ b/eland/filter.py
@@ -14,12 +14,14 @@
 
 # Originally based on code in MIT-licensed pandasticsearch filters
 
+from typing import Dict, Any, List, Optional, Union, cast
+
 
 class BooleanFilter:
-    def __init__(self, *args):
-        self._filter = None
+    def __init__(self) -> None:
+        self._filter: Dict[str, Any] = {}
 
-    def __and__(self, x):
+    def __and__(self, x: "BooleanFilter") -> "BooleanFilter":
         # Combine results
         if isinstance(self, AndFilter):
             if "must_not" in x.subtree:
@@ -37,7 +39,7 @@ class BooleanFilter:
             return x
         return AndFilter(self, x)
 
-    def __or__(self, x):
+    def __or__(self, x: "BooleanFilter") -> "BooleanFilter":
         # Combine results
         if isinstance(self, OrFilter):
             if "must_not" in x.subtree:
@@ -53,85 +55,79 @@ class BooleanFilter:
             return x
         return OrFilter(self, x)
 
-    def __invert__(self):
+    def __invert__(self) -> "BooleanFilter":
         return NotFilter(self)
 
-    def empty(self):
-        if self._filter is None:
-            return True
-        return False
+    def empty(self) -> bool:
+        return not bool(self._filter)
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return str(self._filter)
 
     @property
-    def subtree(self):
+    def subtree(self) -> Dict[str, Any]:
         if "bool" in self._filter:
-            return self._filter["bool"]
+            return cast(Dict[str, Any], self._filter["bool"])
         else:
             return self._filter
 
-    def build(self):
+    def build(self) -> Dict[str, Any]:
         return self._filter
 
 
 # Binary operator
 class AndFilter(BooleanFilter):
-    def __init__(self, *args):
-        [isinstance(x, BooleanFilter) for x in args]
+    def __init__(self, *args: BooleanFilter) -> None:
         super().__init__()
         self._filter = {"bool": {"must": [x.build() for x in args]}}
 
 
 class OrFilter(BooleanFilter):
-    def __init__(self, *args):
-        [isinstance(x, BooleanFilter) for x in args]
+    def __init__(self, *args: BooleanFilter) -> None:
         super().__init__()
         self._filter = {"bool": {"should": [x.build() for x in args]}}
 
 
 class NotFilter(BooleanFilter):
-    def __init__(self, x):
-        assert isinstance(x, BooleanFilter)
+    def __init__(self, x: BooleanFilter) -> None:
         super().__init__()
         self._filter = {"bool": {"must_not": x.build()}}
 
 
 # LeafBooleanFilter
 class GreaterEqual(BooleanFilter):
-    def __init__(self, field, value):
+    def __init__(self, field: str, value: Any) -> None:
         super().__init__()
         self._filter = {"range": {field: {"gte": value}}}
 
 
 class Greater(BooleanFilter):
-    def __init__(self, field, value):
+    def __init__(self, field: str, value: Any) -> None:
         super().__init__()
         self._filter = {"range": {field: {"gt": value}}}
 
 
 class LessEqual(BooleanFilter):
-    def __init__(self, field, value):
+    def __init__(self, field: str, value: Any) -> None:
         super().__init__()
         self._filter = {"range": {field: {"lte": value}}}
 
 
 class Less(BooleanFilter):
-    def __init__(self, field, value):
+    def __init__(self, field: str, value: Any) -> None:
         super().__init__()
         self._filter = {"range": {field: {"lt": value}}}
 
 
 class Equal(BooleanFilter):
-    def __init__(self, field, value):
+    def __init__(self, field: str, value: Any) -> None:
         super().__init__()
         self._filter = {"term": {field: value}}
 
 
 class IsIn(BooleanFilter):
-    def __init__(self, field, value):
+    def __init__(self, field: str, value: List[Any]) -> None:
         super().__init__()
-        assert isinstance(value, list)
         if field == "ids":
             self._filter = {"ids": {"values": value}}
         else:
@@ -139,39 +135,44 @@ class IsIn(BooleanFilter):
 
 
 class Like(BooleanFilter):
-    def __init__(self, field, value):
+    def __init__(self, field: str, value: str) -> None:
         super().__init__()
         self._filter = {"wildcard": {field: value}}
 
 
 class Rlike(BooleanFilter):
-    def __init__(self, field, value):
+    def __init__(self, field: str, value: str) -> None:
         super().__init__()
         self._filter = {"regexp": {field: value}}
 
 
 class Startswith(BooleanFilter):
-    def __init__(self, field, value):
+    def __init__(self, field: str, value: str) -> None:
         super().__init__()
         self._filter = {"prefix": {field: value}}
 
 
 class IsNull(BooleanFilter):
-    def __init__(self, field):
+    def __init__(self, field: str) -> None:
         super().__init__()
         self._filter = {"missing": {"field": field}}
 
 
 class NotNull(BooleanFilter):
-    def __init__(self, field):
+    def __init__(self, field: str) -> None:
         super().__init__()
         self._filter = {"exists": {"field": field}}
 
 
 class ScriptFilter(BooleanFilter):
-    def __init__(self, inline, lang=None, params=None):
+    def __init__(
+        self,
+        inline: str,
+        lang: Optional[str] = None,
+        params: Optional[Dict[str, Any]] = None,
+    ) -> None:
         super().__init__()
-        script = {"source": inline}
+        script: Dict[str, Union[str, Dict[str, Any]]] = {"source": inline}
         if lang is not None:
             script["lang"] = lang
         if params is not None:
@@ -180,6 +181,6 @@ class ScriptFilter(BooleanFilter):
 
 
 class QueryFilter(BooleanFilter):
-    def __init__(self, query):
+    def __init__(self, query: Dict[str, Any]) -> None:
         super().__init__()
         self._filter = query
diff --git a/eland/index.py b/eland/index.py
index 846c2e8..042370e 100644
--- a/eland/index.py
+++ b/eland/index.py
@@ -13,6 +13,12 @@
 # limitations under the License.
 
 
+from typing import Optional, TextIO, TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from .query_compiler import QueryCompiler
+
+
 class Index:
     """
     The index for an eland.DataFrame.
@@ -33,30 +39,36 @@ class Index:
     ID_INDEX_FIELD = "_id"
     ID_SORT_FIELD = "_doc"  # if index field is _id, sort by _doc
 
-    def __init__(self, query_compiler, index_field=None):
+    def __init__(
+        self, query_compiler: "QueryCompiler", index_field: Optional[str] = None
+    ):
         self._query_compiler = query_compiler
 
         # _is_source_field is set immediately within
         # index_field.setter
         self._is_source_field = False
-        self.index_field = index_field
+
+        # The type:ignore is due to mypy not being smart enough
+        # to recognize the property.setter has a different type
+        # than the property.getter.
+        self.index_field = index_field  # type: ignore
 
     @property
-    def sort_field(self):
+    def sort_field(self) -> str:
         if self._index_field == self.ID_INDEX_FIELD:
             return self.ID_SORT_FIELD
         return self._index_field
 
     @property
-    def is_source_field(self):
+    def is_source_field(self) -> bool:
         return self._is_source_field
 
     @property
-    def index_field(self):
+    def index_field(self) -> str:
         return self._index_field
 
     @index_field.setter
-    def index_field(self, index_field):
+    def index_field(self, index_field: Optional[str]) -> None:
         if index_field is None or index_field == Index.ID_INDEX_FIELD:
             self._index_field = Index.ID_INDEX_FIELD
             self._is_source_field = False
@@ -64,18 +76,18 @@ class Index:
             self._index_field = index_field
             self._is_source_field = True
 
-    def __len__(self):
+    def __len__(self) -> int:
         return self._query_compiler._index_count()
 
     # Make iterable
-    def __next__(self):
+    def __next__(self) -> None:
         # TODO resolve this hack to make this 'iterable'
         raise StopIteration()
 
-    def __iter__(self):
+    def __iter__(self) -> "Index":
         return self
 
-    def info_es(self, buf):
+    def info_es(self, buf: TextIO) -> None:
         buf.write("Index:\n")
         buf.write(f" index_field: {self.index_field}\n")
         buf.write(f" is_source_field: {self.is_source_field}\n")
diff --git a/eland/operations.py b/eland/operations.py
index aa344ff..5c85d82 100644
--- a/eland/operations.py
+++ b/eland/operations.py
@@ -14,6 +14,7 @@
 
 import copy
 import warnings
+from typing import Optional
 
 import numpy as np
 
@@ -98,7 +99,7 @@ class Operations:
         else:
             self._arithmetic_op_fields_task.update(display_name, arithmetic_series)
 
-    def get_arithmetic_op_fields(self):
+    def get_arithmetic_op_fields(self) -> Optional[ArithmeticOpFieldsTask]:
         # get an ArithmeticOpFieldsTask if it exists
         return self._arithmetic_op_fields_task
 
diff --git a/eland/query.py b/eland/query.py
index f695deb..ce712b0 100644
--- a/eland/query.py
+++ b/eland/query.py
@@ -14,6 +14,7 @@
 
 import warnings
 from copy import deepcopy
+from typing import Optional, Dict, List, Any
 
 from eland.filter import BooleanFilter, NotNull, IsNull, IsIn
 
@@ -23,7 +24,11 @@ class Query:
     Simple class to manage building Elasticsearch queries.
     """
 
-    def __init__(self, query=None):
+    def __init__(self, query: Optional["Query"] = None):
+        # type defs
+        self._query: BooleanFilter
+        self._aggs: Dict[str, Any]
+
         if query is None:
             self._query = BooleanFilter()
             self._aggs = {}
@@ -32,7 +37,7 @@ class Query:
             self._query = deepcopy(query._query)
             self._aggs = deepcopy(query._aggs)
 
-    def exists(self, field, must=True):
+    def exists(self, field: str, must: bool = True) -> None:
         """
         Add exists query
         https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-exists-query.html
@@ -48,7 +53,7 @@ class Query:
             else:
                 self._query = self._query & IsNull(field)
 
-    def ids(self, items, must=True):
+    def ids(self, items: List[Any], must: bool = True) -> None:
         """
         Add ids query
         https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-ids-query.html
@@ -64,7 +69,7 @@ class Query:
             else:
                 self._query = self._query & ~(IsIn("ids", items))
 
-    def terms(self, field, items, must=True):
+    def terms(self, field: str, items: List[str], must: bool = True) -> None:
         """
         Add ids query
         https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-terms-query.html
@@ -80,7 +85,7 @@ class Query:
             else:
                 self._query = self._query & ~(IsIn(field, items))
 
-    def terms_aggs(self, name, func, field, es_size):
+    def terms_aggs(self, name: str, func: str, field: str, es_size: int) -> None:
         """
         Add terms agg e.g
 
@@ -96,7 +101,7 @@ class Query:
         agg = {func: {"field": field, "size": es_size}}
         self._aggs[name] = agg
 
-    def metric_aggs(self, name, func, field):
+    def metric_aggs(self, name: str, func: str, field: str) -> None:
         """
         Add metric agg e.g
 
@@ -111,7 +116,14 @@ class Query:
         agg = {func: {"field": field}}
         self._aggs[name] = agg
 
-    def hist_aggs(self, name, field, min_aggs, max_aggs, num_bins):
+    def hist_aggs(
+        self,
+        name: str,
+        field: str,
+        min_aggs: Dict[str, Any],
+        max_aggs: Dict[str, Any],
+        num_bins: int,
+    ) -> None:
         """
         Add histogram agg e.g.
         "aggs": {
@@ -127,14 +139,15 @@ class Query:
         max = max_aggs[field]
 
         interval = (max - min) / num_bins
-        offset = min
-
-        agg = {"histogram": {"field": field, "interval": interval, "offset": offset}}
 
         if interval != 0:
+            offset = min
+            agg = {
+                "histogram": {"field": field, "interval": interval, "offset": offset}
+            }
             self._aggs[name] = agg
 
-    def to_search_body(self):
+    def to_search_body(self) -> Dict[str, Any]:
         body = {}
         if self._aggs:
             body["aggs"] = self._aggs
@@ -142,19 +155,19 @@ class Query:
             body["query"] = self._query.build()
         return body
 
-    def to_count_body(self):
+    def to_count_body(self) -> Optional[Dict[str, Any]]:
         if len(self._aggs) > 0:
-            warnings.warn("Requesting count for agg query {}", self)
+            warnings.warn(f"Requesting count for agg query {self}")
         if self._query.empty():
             return None
         else:
             return {"query": self._query.build()}
 
-    def update_boolean_filter(self, boolean_filter):
+    def update_boolean_filter(self, boolean_filter: BooleanFilter) -> None:
         if self._query.empty():
             self._query = boolean_filter
         else:
             self._query = self._query & boolean_filter
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return repr(self.to_search_body())
diff --git a/eland/query_compiler.py b/eland/query_compiler.py
index 2a4be4d..5d55076 100644
--- a/eland/query_compiler.py
+++ b/eland/query_compiler.py
@@ -14,6 +14,7 @@
 
 import copy
 from datetime import datetime
+from typing import Optional, TYPE_CHECKING
 
 import numpy as np
 import pandas as pd
@@ -28,6 +29,9 @@ from eland.common import (
     elasticsearch_date_to_pandas_date,
 )
 
+if TYPE_CHECKING:
+    from .tasks import ArithmeticOpFieldsTask  # noqa: F401
+
 
 class QueryCompiler:
     """
@@ -348,7 +352,7 @@ class QueryCompiler:
 
         return out
 
-    def _index_count(self):
+    def _index_count(self) -> int:
         """
         Returns
         -------
@@ -562,10 +566,10 @@ class QueryCompiler:
 
         return result
 
-    def get_arithmetic_op_fields(self):
+    def get_arithmetic_op_fields(self) -> Optional["ArithmeticOpFieldsTask"]:
         return self._operations.get_arithmetic_op_fields()
 
-    def display_name_to_aggregatable_name(self, display_name):
+    def display_name_to_aggregatable_name(self, display_name: str) -> str:
         aggregatable_field_name = self._mappings.aggregatable_field_name(display_name)
 
         return aggregatable_field_name
diff --git a/eland/tasks.py b/eland/tasks.py
index aa7b158..4086942 100644
--- a/eland/tasks.py
+++ b/eland/tasks.py
@@ -13,12 +13,22 @@
 # limitations under the License.
 
 from abc import ABC, abstractmethod
+from typing import TYPE_CHECKING, List, Dict, Any, Tuple
 
 from eland import SortOrder
 from eland.actions import HeadAction, TailAction, SortIndexAction
 from eland.arithmetics import ArithmeticSeries
 
 
+if TYPE_CHECKING:
+    from .actions import PostProcessingAction  # noqa: F401
+    from .filter import BooleanFilter  # noqa: F401
+    from .query_compiler import QueryCompiler  # noqa: F401
+
+QUERY_PARAMS_TYPE = Dict[str, Any]
+RESOLVED_TASK_TYPE = Tuple[QUERY_PARAMS_TYPE, List["PostProcessingAction"]]
+
+
 class Task(ABC):
     """
     Abstract class for tasks
@@ -29,44 +39,51 @@ class Task(ABC):
             The task type (e.g. head, tail etc.)
     """
 
-    def __init__(self, task_type):
+    def __init__(self, task_type: str):
         self._task_type = task_type
 
     @property
-    def type(self):
+    def type(self) -> str:
         return self._task_type
 
     @abstractmethod
-    def resolve_task(self, query_params, post_processing, query_compiler):
+    def resolve_task(
+        self,
+        query_params: QUERY_PARAMS_TYPE,
+        post_processing: List["PostProcessingAction"],
+        query_compiler: "QueryCompiler",
+    ) -> RESOLVED_TASK_TYPE:
         pass
 
     @abstractmethod
-    def __repr__(self):
+    def __repr__(self) -> str:
         pass
 
 
 class SizeTask(Task):
-    def __init__(self, task_type):
-        super().__init__(task_type)
-
     @abstractmethod
-    def size(self):
+    def size(self) -> int:
         # must override
         pass
 
 
 class HeadTask(SizeTask):
-    def __init__(self, sort_field, count):
+    def __init__(self, sort_field: str, count: int):
         super().__init__("head")
 
         # Add a task that is an ascending sort with size=count
         self._sort_field = sort_field
         self._count = count
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"('{self._task_type}': ('sort_field': '{self._sort_field}', 'count': {self._count}))"
 
-    def resolve_task(self, query_params, post_processing, query_compiler):
+    def resolve_task(
+        self,
+        query_params: QUERY_PARAMS_TYPE,
+        post_processing: List["PostProcessingAction"],
+        query_compiler: "QueryCompiler",
+    ) -> RESOLVED_TASK_TYPE:
         # head - sort asc, size n
         # |12345-------------|
         query_sort_field = self._sort_field
@@ -97,22 +114,24 @@ class HeadTask(SizeTask):
 
         return query_params, post_processing
 
-    def size(self):
+    def size(self) -> int:
         return self._count
 
 
 class TailTask(SizeTask):
-    def __init__(self, sort_field, count):
+    def __init__(self, sort_field: str, count: int):
         super().__init__("tail")
 
         # Add a task that is descending sort with size=count
         self._sort_field = sort_field
         self._count = count
 
-    def __repr__(self):
-        return f"('{self._task_type}': ('sort_field': '{self._sort_field}', 'count': {self._count}))"
-
-    def resolve_task(self, query_params, post_processing, query_compiler):
+    def resolve_task(
+        self,
+        query_params: QUERY_PARAMS_TYPE,
+        post_processing: List["PostProcessingAction"],
+        query_compiler: "QueryCompiler",
+    ) -> RESOLVED_TASK_TYPE:
         # tail - sort desc, size n, post-process sort asc
         # |-------------12345|
         query_sort_field = self._sort_field
@@ -123,7 +142,10 @@ class TailTask(SizeTask):
         if (
             query_params["query_size"] is not None
             and query_params["query_sort_order"] == query_sort_order
-            and post_processing == ["sort_index"]
+            and (
+                len(post_processing) == 1
+                and isinstance(post_processing[0], SortIndexAction)
+            )
         ):
             if query_size < query_params["query_size"]:
                 query_params["query_size"] = query_size
@@ -156,12 +178,15 @@ class TailTask(SizeTask):
 
         return query_params, post_processing
 
-    def size(self):
+    def size(self) -> int:
         return self._count
 
+    def __repr__(self) -> str:
+        return f"('{self._task_type}': ('sort_field': '{self._sort_field}', 'count': {self._count}))"
+
 
 class QueryIdsTask(Task):
-    def __init__(self, must, ids):
+    def __init__(self, must: bool, ids: List[str]):
         """
         Parameters
         ----------
@@ -176,17 +201,21 @@ class QueryIdsTask(Task):
         self._must = must
         self._ids = ids
 
-    def resolve_task(self, query_params, post_processing, query_compiler):
+    def resolve_task(
+        self,
+        query_params: QUERY_PARAMS_TYPE,
+        post_processing: List["PostProcessingAction"],
+        query_compiler: "QueryCompiler",
+    ) -> RESOLVED_TASK_TYPE:
         query_params["query"].ids(self._ids, must=self._must)
-
         return query_params, post_processing
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"('{self._task_type}': ('must': {self._must}, 'ids': {self._ids}))"
 
 
 class QueryTermsTask(Task):
-    def __init__(self, must, field, terms):
+    def __init__(self, must: bool, field: str, terms: List[Any]):
         """
         Parameters
         ----------
@@ -205,20 +234,24 @@ class QueryTermsTask(Task):
         self._field = field
         self._terms = terms
 
-    def __repr__(self):
+    def resolve_task(
+        self,
+        query_params: QUERY_PARAMS_TYPE,
+        post_processing: List["PostProcessingAction"],
+        query_compiler: "QueryCompiler",
+    ) -> RESOLVED_TASK_TYPE:
+        query_params["query"].terms(self._field, self._terms, must=self._must)
+        return query_params, post_processing
+
+    def __repr__(self) -> str:
         return (
             f"('{self._task_type}': ('must': {self._must}, "
             f"'field': '{self._field}', 'terms': {self._terms}))"
         )
 
-    def resolve_task(self, query_params, post_processing, query_compiler):
-        query_params["query"].terms(self._field, self._terms, must=self._must)
-
-        return query_params, post_processing
-
 
 class BooleanFilterTask(Task):
-    def __init__(self, boolean_filter):
+    def __init__(self, boolean_filter: "BooleanFilter"):
         """
         Parameters
         ----------
@@ -229,17 +262,21 @@ class BooleanFilterTask(Task):
 
         self._boolean_filter = boolean_filter
 
-    def __repr__(self):
-        return f"('{self._task_type}': ('boolean_filter': {self._boolean_filter!r}))"
-
-    def resolve_task(self, query_params, post_processing, query_compiler):
+    def resolve_task(
+        self,
+        query_params: QUERY_PARAMS_TYPE,
+        post_processing: List["PostProcessingAction"],
+        query_compiler: "QueryCompiler",
+    ) -> RESOLVED_TASK_TYPE:
         query_params["query"].update_boolean_filter(self._boolean_filter)
-
         return query_params, post_processing
 
+    def __repr__(self) -> str:
+        return f"('{self._task_type}': ('boolean_filter': {self._boolean_filter!r}))"
+
 
 class ArithmeticOpFieldsTask(Task):
-    def __init__(self, display_name, arithmetic_series):
+    def __init__(self, display_name: str, arithmetic_series: ArithmeticSeries):
         super().__init__("arithmetic_op_fields")
 
         self._display_name = display_name
@@ -248,19 +285,16 @@ class ArithmeticOpFieldsTask(Task):
             raise TypeError(f"Expecting ArithmeticSeries got {type(arithmetic_series)}")
         self._arithmetic_series = arithmetic_series
 
-    def __repr__(self):
-        return (
-            f"('{self._task_type}': ("
-            f"'display_name': {self._display_name}, "
-            f"'arithmetic_object': {self._arithmetic_series}"
-            f"))"
-        )
-
-    def update(self, display_name, arithmetic_series):
+    def update(self, display_name: str, arithmetic_series: ArithmeticSeries) -> None:
         self._display_name = display_name
         self._arithmetic_series = arithmetic_series
 
-    def resolve_task(self, query_params, post_processing, query_compiler):
+    def resolve_task(
+        self,
+        query_params: QUERY_PARAMS_TYPE,
+        post_processing: List["PostProcessingAction"],
+        query_compiler: "QueryCompiler",
+    ) -> RESOLVED_TASK_TYPE:
         # https://www.elastic.co/guide/en/elasticsearch/painless/current/painless-api-reference-shared-java-lang.html#painless-api-reference-shared-Math
         """
         "script_fields": {
@@ -272,7 +306,10 @@ class ArithmeticOpFieldsTask(Task):
         }
         """
         if query_params["query_script_fields"] is None:
-            query_params["query_script_fields"] = dict()
+            query_params["query_script_fields"] = {}
+
+        # TODO: Remove this once 'query_params' becomes a dataclass.
+        assert isinstance(query_params["query_script_fields"], dict)
 
         if self._display_name in query_params["query_script_fields"]:
             raise NotImplementedError(
@@ -286,3 +323,11 @@ class ArithmeticOpFieldsTask(Task):
         }
 
         return query_params, post_processing
+
+    def __repr__(self) -> str:
+        return (
+            f"('{self._task_type}': ("
+            f"'display_name': {self._display_name}, "
+            f"'arithmetic_object': {self._arithmetic_series}"
+            f"))"
+        )
diff --git a/eland/utils.py b/eland/utils.py
index e3e9a8f..37327ae 100644
--- a/eland/utils.py
+++ b/eland/utils.py
@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import csv
+from typing import Union, List, Tuple, Optional, Mapping
 
 import pandas as pd
 from pandas.io.parsers import _c_parser_defaults
@@ -20,10 +21,14 @@ from pandas.io.parsers import _c_parser_defaults
 from eland import DataFrame
 from eland.field_mappings import FieldMappings
 from eland.common import ensure_es_client, DEFAULT_CHUNK_SIZE
+from elasticsearch import Elasticsearch
 from elasticsearch.helpers import bulk
 
 
-def read_es(es_client, es_index_pattern):
+def read_es(
+    es_client: Union[str, List[str], Tuple[str, ...], Elasticsearch],
+    es_index_pattern: str,
+) -> DataFrame:
     """
     Utility method to create an eland.Dataframe from an Elasticsearch index_pattern.
     (Similar to pandas.read_csv, but source data is an Elasticsearch index rather than
@@ -50,16 +55,16 @@ def read_es(es_client, es_index_pattern):
 
 
 def pandas_to_eland(
-    pd_df,
-    es_client,
-    es_dest_index,
-    es_if_exists="fail",
-    es_refresh=False,
-    es_dropna=False,
-    es_type_overrides=None,
-    chunksize=None,
-    use_pandas_index_for_es_ids=True,
-):
+    pd_df: pd.DataFrame,
+    es_client: Union[str, List[str], Tuple[str, ...], Elasticsearch],
+    es_dest_index: str,
+    es_if_exists: str = "fail",
+    es_refresh: bool = False,
+    es_dropna: bool = False,
+    es_type_overrides: Optional[Mapping[str, str]] = None,
+    chunksize: Optional[int] = None,
+    use_pandas_index_for_es_ids: bool = True,
+) -> DataFrame:
     """
     Append a pandas DataFrame to an Elasticsearch index.
     Mainly used in testing.
@@ -217,7 +222,7 @@ def pandas_to_eland(
     return DataFrame(es_client, es_dest_index)
 
 
-def eland_to_pandas(ed_df, show_progress=False):
+def eland_to_pandas(ed_df: DataFrame, show_progress: bool = False) -> pd.DataFrame:
     """
     Convert an eland.Dataframe to a pandas.DataFrame
 
diff --git a/noxfile.py b/noxfile.py
index ff80ad8..2d31e48 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -1,4 +1,5 @@
 import os
+import subprocess
 from pathlib import Path
 import nox
 import elasticsearch
@@ -12,6 +13,19 @@ SOURCE_FILES = (
     "docs/",
 )
 
+# Whenever type-hints are completed on a file it should
+# be added here so that this file will continue to be checked
+# by mypy. Errors from other files are ignored.
+TYPED_FILES = {
+    "eland/actions.py",
+    "eland/arithmetics.py",
+    "eland/common.py",
+    "eland/filter.py",
+    "eland/index.py",
+    "eland/query.py",
+    "eland/tasks.py",
+}
+
 
 @nox.session(reuse_venv=True)
 def blacken(session):
@@ -22,10 +36,28 @@ def blacken(session):
 
 @nox.session(reuse_venv=True)
 def lint(session):
-    session.install("black", "flake8")
+    session.install("black", "flake8", "mypy")
     session.run("black", "--check", "--target-version=py36", *SOURCE_FILES)
     session.run("flake8", "--ignore=E501,W503,E402,E712", *SOURCE_FILES)
 
+    # TODO: When all files are typed we can change this to .run("mypy", "--strict", "eland/")
+    session.log("mypy --strict eland/")
+    for typed_file in TYPED_FILES:
+        popen = subprocess.Popen(
+            f"mypy --strict {typed_file}",
+            shell=True,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.STDOUT,
+        )
+        popen.wait()
+        errors = []
+        for line in popen.stdout.read().decode().split("\n"):
+            filepath = line.partition(":")[0]
+            if filepath in TYPED_FILES:
+                errors.append(line)
+        if errors:
+            session.error("\n" + "\n".join(sorted(set(errors))))
+
 
 @nox.session(python=["3.6", "3.7", "3.8"])
 def test(session):