diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 8592911..a285a2d 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -40,6 +40,11 @@ jobs:
     - name: Tests
       run: |
         pytest --cov=snowplow_tracker --cov-report=xml
+
+    - name: MyPy
+      run: |
+        python -m pip install mypy
+        mypy snowplow_tracker --exclude '/test'
       
     - name: Demo
       run: |
diff --git a/setup.py b/setup.py
index d0ef7f0..e1b2aa3 100644
--- a/setup.py
+++ b/setup.py
@@ -65,5 +65,9 @@
         "Programming Language :: Python :: 3.12",
         "Operating System :: OS Independent",
     ],
-    install_requires=["requests>=2.25.1,<3.0", "typing_extensions>=3.7.4"],
+    install_requires=[
+        "requests>=2.25.1,<3.0",
+        "types-requests>=2.25.1,<3.0",
+        "typing_extensions>=3.7.4",
+    ],
 )
diff --git a/snowplow_tracker/constants.py b/snowplow_tracker/constants.py
index 579ff86..53ecc15 100644
--- a/snowplow_tracker/constants.py
+++ b/snowplow_tracker/constants.py
@@ -18,7 +18,7 @@
 from snowplow_tracker import _version, SelfDescribingJson
 
 VERSION = "py-%s" % _version.__version__
-DEFAULT_ENCODE_BASE64 = True
+DEFAULT_ENCODE_BASE64: bool = True  # Type hint required for Python 3.6 MyPy check
 BASE_SCHEMA_PATH = "iglu:com.snowplowanalytics.snowplow"
 MOBILE_SCHEMA_PATH = "iglu:com.snowplowanalytics.mobile"
 SCHEMA_TAG = "jsonschema"
diff --git a/snowplow_tracker/contracts.py b/snowplow_tracker/contracts.py
index c54ac66..3b17e1a 100644
--- a/snowplow_tracker/contracts.py
+++ b/snowplow_tracker/contracts.py
@@ -77,7 +77,7 @@ def _get_parameter_name() -> str:
     match = _MATCH_FIRST_PARAMETER_REGEX.search(code)
     if not match:
         return "Unnamed parameter"
-    return match.groups(0)[0]
+    return str(match.groups(0)[0])
 
 
 def _check_form_element(element: Dict[str, Any]) -> bool:
diff --git a/snowplow_tracker/emitters.py b/snowplow_tracker/emitters.py
index af23356..6a138f0 100644
--- a/snowplow_tracker/emitters.py
+++ b/snowplow_tracker/emitters.py
@@ -20,7 +20,7 @@
 import threading
 import requests
 import random
-from typing import Optional, Union, Tuple, Dict
+from typing import Optional, Union, Tuple, Dict, cast, Callable
 from queue import Queue
 
 from snowplow_tracker.self_describing_json import SelfDescribingJson
@@ -31,6 +31,7 @@
     Method,
     SuccessCallback,
     FailureCallback,
+    EmitterProtocol,
 )
 from snowplow_tracker.contracts import one_of
 from snowplow_tracker.event_store import EventStore, InMemoryEventStore
@@ -48,7 +49,20 @@
 METHODS = {"get", "post"}
 
 
-class Emitter(object):
+# Unifes the two request methods under one interface
+class Requester:
+    post: Callable
+    get: Callable
+
+    def __init__(self, post: Callable, get: Callable):
+        # 3.6 MyPy compatibility:
+        # error: Cannot assign to a method
+        # https://github.com/python/mypy/issues/2427
+        setattr(self, "post", post)
+        setattr(self, "get", get)
+
+
+class Emitter(EmitterProtocol):
     """
     Synchronously send Snowplow events to a Snowplow collector
     Supports both GET and POST requests
@@ -151,12 +165,15 @@ def __init__(
         self.retry_timer = FlushTimer(emitter=self, repeating=False)
 
         self.max_retry_delay_seconds = max_retry_delay_seconds
-        self.retry_delay = 0
+        self.retry_delay: Union[int, float] = 0
 
         self.custom_retry_codes = custom_retry_codes
         logger.info("Emitter initialized with endpoint " + self.endpoint)
 
-        self.request_method = requests if session is None else session
+        if session is None:
+            self.request_method = Requester(post=requests.post, get=requests.get)
+        else:
+            self.request_method = Requester(post=session.post, get=session.get)
 
     @staticmethod
     def as_collector_uri(
@@ -183,7 +200,7 @@ def as_collector_uri(
 
         if endpoint.split("://")[0] in PROTOCOLS:
             endpoint_arr = endpoint.split("://")
-            protocol = endpoint_arr[0]
+            protocol = cast(HttpProtocol, endpoint_arr[0])
             endpoint = endpoint_arr[1]
 
         if method == "get":
@@ -427,6 +444,10 @@ def _cancel_retry_timer(self) -> None:
         """
         self.retry_timer.cancel()
 
+    # This is only here to satisfy the `EmitterProtocol` interface
+    def async_flush(self) -> None:
+        return
+
 
 class AsyncEmitter(Emitter):
     """
@@ -446,7 +467,7 @@ def __init__(
         byte_limit: Optional[int] = None,
         request_timeout: Optional[Union[float, Tuple[float, float]]] = None,
         max_retry_delay_seconds: int = 60,
-        buffer_capacity: int = None,
+        buffer_capacity: Optional[int] = None,
         custom_retry_codes: Dict[int, bool] = {},
         event_store: Optional[EventStore] = None,
         session: Optional[requests.Session] = None,
@@ -501,7 +522,7 @@ def __init__(
             event_store=event_store,
             session=session,
         )
-        self.queue = Queue()
+        self.queue: Queue = Queue()
         for i in range(thread_count):
             t = threading.Thread(target=self.consume)
             t.daemon = True
diff --git a/snowplow_tracker/event_store.py b/snowplow_tracker/event_store.py
index 898f92f..b8d1302 100644
--- a/snowplow_tracker/event_store.py
+++ b/snowplow_tracker/event_store.py
@@ -15,6 +15,7 @@
 #     language governing permissions and limitations there under.
 # """
 
+from typing import List
 from typing_extensions import Protocol
 from snowplow_tracker.typing import PayloadDict, PayloadDictList
 from logging import Logger
@@ -25,7 +26,7 @@ class EventStore(Protocol):
     EventStore protocol. For buffering events in the Emitter.
     """
 
-    def add_event(payload: PayloadDict) -> bool:
+    def add_event(self, payload: PayloadDict) -> bool:
         """
         Add PayloadDict to buffer. Returns True if successful.
 
@@ -35,7 +36,7 @@ def add_event(payload: PayloadDict) -> bool:
         """
         ...
 
-    def get_events_batch() -> PayloadDictList:
+    def get_events_batch(self) -> PayloadDictList:
         """
         Get a list of all the PayloadDicts in the buffer.
 
@@ -43,7 +44,7 @@ def get_events_batch() -> PayloadDictList:
         """
         ...
 
-    def cleanup(batch: PayloadDictList, need_retry: bool) -> None:
+    def cleanup(self, batch: PayloadDictList, need_retry: bool) -> None:
         """
         Removes sent events from the event store. If events need to be retried they are re-added to the buffer.
 
@@ -54,7 +55,7 @@ def cleanup(batch: PayloadDictList, need_retry: bool) -> None:
         """
         ...
 
-    def size() -> int:
+    def size(self) -> int:
         """
         Returns the number of events in the buffer
 
@@ -76,7 +77,7 @@ def __init__(self, logger: Logger, buffer_capacity: int = 10000) -> None:
                                     When the buffer is full new events are lost.
         :type   buffer_capacity     int
         """
-        self.event_buffer = []
+        self.event_buffer: List[PayloadDict] = []
         self.buffer_capacity = buffer_capacity
         self.logger = logger
 
diff --git a/snowplow_tracker/events/event.py b/snowplow_tracker/events/event.py
index c9d9b82..fb300b8 100644
--- a/snowplow_tracker/events/event.py
+++ b/snowplow_tracker/events/event.py
@@ -97,10 +97,9 @@ def build_payload(
         if self.event_subject is not None:
             fin_payload_dict = self.event_subject.combine_subject(subject)
         else:
-            fin_payload_dict = None if subject is None else subject.standard_nv_pairs
+            fin_payload_dict = {} if subject is None else subject.standard_nv_pairs
 
-        if fin_payload_dict is not None:
-            self.payload.add_dict(fin_payload_dict)
+        self.payload.add_dict(fin_payload_dict)
         return self.payload
 
     @property
diff --git a/snowplow_tracker/events/screen_view.py b/snowplow_tracker/events/screen_view.py
index d0cea5d..6b4af92 100644
--- a/snowplow_tracker/events/screen_view.py
+++ b/snowplow_tracker/events/screen_view.py
@@ -15,7 +15,7 @@
 #     language governing permissions and limitations there under.
 # """
 
-from typing import Optional, List
+from typing import Dict, Optional, List
 from snowplow_tracker.typing import JsonEncoderFunction
 from snowplow_tracker.events.event import Event
 from snowplow_tracker.events.self_describing import SelfDescribing
@@ -76,7 +76,7 @@ def __init__(
         super(ScreenView, self).__init__(
             event_subject=event_subject, context=context, true_timestamp=true_timestamp
         )
-        self.screen_view_properties = {}
+        self.screen_view_properties: Dict[str, str] = {}
         self.id_ = id_
         self.name = name
         self.type = type
diff --git a/snowplow_tracker/events/structured_event.py b/snowplow_tracker/events/structured_event.py
index 00658e9..23abafa 100644
--- a/snowplow_tracker/events/structured_event.py
+++ b/snowplow_tracker/events/structured_event.py
@@ -15,7 +15,7 @@
 #     language governing permissions and limitations there under.
 # """
 from snowplow_tracker.events.event import Event
-from typing import Optional, List
+from typing import Optional, List, Union
 from snowplow_tracker.subject import Subject
 from snowplow_tracker.self_describing_json import SelfDescribingJson
 from snowplow_tracker.contracts import non_empty_string
@@ -41,7 +41,7 @@ def __init__(
         action: str,
         label: Optional[str] = None,
         property_: Optional[str] = None,
-        value: Optional[int] = None,
+        value: Optional[Union[int, float]] = None,
         event_subject: Optional[Subject] = None,
         context: Optional[List[SelfDescribingJson]] = None,
         true_timestamp: Optional[float] = None,
@@ -84,7 +84,7 @@ def category(self) -> Optional[str]:
         return self.payload.nv_pairs.get("se_ca")
 
     @category.setter
-    def category(self, value: Optional[str]):
+    def category(self, value: str):
         non_empty_string(value)
         self.payload.add("se_ca", value)
 
@@ -96,7 +96,7 @@ def action(self) -> Optional[str]:
         return self.payload.nv_pairs.get("se_ac")
 
     @action.setter
-    def action(self, value: Optional[str]):
+    def action(self, value: str):
         non_empty_string(value)
         self.payload.add("se_ac", value)
 
@@ -123,12 +123,12 @@ def property_(self, value: Optional[str]):
         self.payload.add("se_pr", value)
 
     @property
-    def value(self) -> Optional[int]:
+    def value(self) -> Optional[Union[int, float]]:
         """
         A value associated with the user action
         """
         return self.payload.nv_pairs.get("se_va")
 
     @value.setter
-    def value(self, value: Optional[int]):
+    def value(self, value: Optional[Union[int, float]]):
         self.payload.add("se_va", value)
diff --git a/snowplow_tracker/payload.py b/snowplow_tracker/payload.py
index 26e3262..18d1bf4 100644
--- a/snowplow_tracker/payload.py
+++ b/snowplow_tracker/payload.py
@@ -83,9 +83,8 @@ def add_json(
 
             if encode_base64:
                 encoded_dict = base64.urlsafe_b64encode(json_dict.encode("utf-8"))
-                if not isinstance(encoded_dict, str):
-                    encoded_dict = encoded_dict.decode("utf-8")
-                self.add(type_when_encoded, encoded_dict)
+                encoded_dict_str = encoded_dict.decode("utf-8")
+                self.add(type_when_encoded, encoded_dict_str)
 
             else:
                 self.add(type_when_not_encoded, json_dict)
diff --git a/snowplow_tracker/snowplow.py b/snowplow_tracker/snowplow.py
index d824ed2..daa1434 100644
--- a/snowplow_tracker/snowplow.py
+++ b/snowplow_tracker/snowplow.py
@@ -16,7 +16,7 @@
 # """
 
 import logging
-from typing import Optional
+from typing import Dict, Optional
 from snowplow_tracker import (
     Tracker,
     Emitter,
@@ -37,7 +37,7 @@
 
 
 class Snowplow:
-    _trackers = {}
+    _trackers: Dict[str, Tracker] = {}
 
     @staticmethod
     def create_tracker(
@@ -149,7 +149,7 @@ def reset(cls):
         cls._trackers = {}
 
     @classmethod
-    def get_tracker(cls, namespace: str) -> Tracker:
+    def get_tracker(cls, namespace: str) -> Optional[Tracker]:
         """
         Returns a Snowplow tracker from the Snowplow object if it exists
         :param  namespace:              Snowplow tracker namespace
diff --git a/snowplow_tracker/subject.py b/snowplow_tracker/subject.py
index c3165d3..cbf29aa 100644
--- a/snowplow_tracker/subject.py
+++ b/snowplow_tracker/subject.py
@@ -15,7 +15,7 @@
 #     language governing permissions and limitations there under.
 # """
 
-from typing import Optional
+from typing import Dict, Optional, Union
 from snowplow_tracker.contracts import one_of, greater_than
 from snowplow_tracker.typing import SupportedPlatform, SUPPORTED_PLATFORMS, PayloadDict
 
@@ -30,7 +30,7 @@ class Subject(object):
     """
 
     def __init__(self) -> None:
-        self.standard_nv_pairs = {"p": DEFAULT_PLATFORM}
+        self.standard_nv_pairs: Dict[str, Union[str, int]] = {"p": DEFAULT_PLATFORM}
 
     def set_platform(self, value: SupportedPlatform) -> "Subject":
         """
diff --git a/snowplow_tracker/tracker.py b/snowplow_tracker/tracker.py
index 2effe83..4dc489d 100644
--- a/snowplow_tracker/tracker.py
+++ b/snowplow_tracker/tracker.py
@@ -80,13 +80,13 @@ def __init__(
         if subject is None:
             subject = Subject()
 
-        if type(emitters) is list:
+        if isinstance(emitters, list):
             non_empty(emitters)
             self.emitters = emitters
         else:
             self.emitters = [emitters]
 
-        self.subject = subject
+        self.subject: Optional[Subject] = subject
         self.encode_base64 = encode_base64
         self.json_encoder = json_encoder
 
@@ -145,6 +145,8 @@ def track(
         if "eid" in payload.nv_pairs.keys():
             return payload.nv_pairs["eid"]
 
+        return None
+
     def complete_payload(
         self,
         event: Event,
@@ -298,7 +300,7 @@ def track_link_click(
         )
         non_empty_string(target_url)
 
-        properties = {}
+        properties: Dict[str, Union[str, ElementClasses]] = {}
         properties["targetUrl"] = target_url
         if element_id is not None:
             properties["elementId"] = element_id
@@ -361,7 +363,7 @@ def track_add_to_cart(
         )
         non_empty_string(sku)
 
-        properties = {}
+        properties: Union[Dict[str, Union[str, float, int]]] = {}
         properties["sku"] = sku
         properties["quantity"] = quantity
         if name is not None:
@@ -425,7 +427,7 @@ def track_remove_from_cart(
         )
         non_empty_string(sku)
 
-        properties = {}
+        properties: Dict[str, Union[str, float, int]] = {}
         properties["sku"] = sku
         properties["quantity"] = quantity
         if name is not None:
@@ -493,7 +495,7 @@ def track_form_change(
         if type_ is not None:
             one_of(type_.lower(), FORM_TYPES)
 
-        properties = dict()
+        properties: Dict[str, Union[Optional[str], ElementClasses]] = dict()
         properties["formId"] = form_id
         properties["elementId"] = element_id
         properties["nodeName"] = node_name
@@ -549,7 +551,9 @@ def track_form_submit(
         for element in elements or []:
             form_element(element)
 
-        properties = dict()
+        properties: Dict[
+            str, Union[str, ElementClasses, FormClasses, List[Dict[str, Any]]]
+        ] = dict()
         properties["formId"] = form_id
         if form_classes is not None:
             properties["formClasses"] = form_classes
@@ -602,7 +606,9 @@ def track_site_search(
         )
         non_empty(terms)
 
-        properties = {}
+        properties: Dict[
+            str, Union[Sequence[str], Dict[str, Union[str, bool]], int]
+        ] = {}
         properties["terms"] = terms
         if filters is not None:
             properties["filters"] = filters
@@ -878,7 +884,7 @@ def track_struct_event(
         action: str,
         label: Optional[str] = None,
         property_: Optional[str] = None,
-        value: Optional[float] = None,
+        value: Optional[Union[int, float]] = None,
         context: Optional[List[SelfDescribingJson]] = None,
         tstamp: Optional[float] = None,
         event_subject: Optional[Subject] = None,
@@ -1037,4 +1043,9 @@ def add_emitter(self, emitter: EmitterProtocol) -> "Tracker":
         return self
 
     def get_namespace(self) -> str:
-        return self.standard_nv_pairs["tna"]
+        # As app_id is added to the standard_nv_pairs dict above with a type of Optional[str], the type for
+        # the whole standard_nv_pairs dict is inferred to be dict[str, Optional[str]].
+        # But, we know that "tna" should always be present in the dict, since namespace is a required argument.
+        #
+        # This ignores MyPy saying Incompatible return value type (got "str | None", expected "str")
+        return self.standard_nv_pairs["tna"]  # type: ignore
diff --git a/snowplow_tracker/tracker_configuration.py b/snowplow_tracker/tracker_configuration.py
index af2a4b9..6a574dc 100644
--- a/snowplow_tracker/tracker_configuration.py
+++ b/snowplow_tracker/tracker_configuration.py
@@ -22,7 +22,7 @@
 class TrackerConfiguration(object):
     def __init__(
         self,
-        encode_base64: Optional[bool] = None,
+        encode_base64: bool = True,
         json_encoder: Optional[JsonEncoderFunction] = None,
     ) -> None:
         """
@@ -37,18 +37,16 @@ def __init__(
         self.json_encoder = json_encoder
 
     @property
-    def encode_base64(self) -> Optional[bool]:
+    def encode_base64(self) -> bool:
         """
         Whether JSONs in the payload should be base-64 encoded. Default is True.
         """
         return self._encode_base64
 
     @encode_base64.setter
-    def encode_base64(self, value: Optional[bool]):
+    def encode_base64(self, value: bool):
         if isinstance(value, bool) or value is None:
             self._encode_base64 = value
-        else:
-            raise ValueError("encode_base64 must be True or False")
 
     @property
     def json_encoder(self) -> Optional[JsonEncoderFunction]:
diff --git a/snowplow_tracker/typing.py b/snowplow_tracker/typing.py
index 5bbc477..3e97356 100644
--- a/snowplow_tracker/typing.py
+++ b/snowplow_tracker/typing.py
@@ -65,5 +65,10 @@
 
 
 class EmitterProtocol(Protocol):
-    def input(self, payload: PayloadDict) -> None:
-        ...
+    def input(self, payload: PayloadDict) -> None: ...
+
+    def flush(self) -> None: ...
+
+    def async_flush(self) -> None: ...
+
+    def sync_flush(self) -> None: ...