diff --git a/crashreport_stats/management/commands/stats.py b/crashreport_stats/management/commands/stats.py
index c30c2e0..35133c5 100644
--- a/crashreport_stats/management/commands/stats.py
+++ b/crashreport_stats/management/commands/stats.py
@@ -4,10 +4,11 @@
 heartbeats, crashes, and versions sent from Hiccup clients.
 """
 import datetime
+from typing import Dict, List, Optional
 
 from django.core.management.base import BaseCommand
 from django.db import transaction
-from django.db.models import Count, F, Q
+from django.db.models import Count, F, Model, Q, QuerySet
 from django.db.models.functions import TruncDate
 import pytz
 
@@ -17,6 +18,8 @@
     StatsMetadata,
     Version,
     VersionDaily,
+    _VersionStats,
+    _DailyVersionStats,
 )
 from crashreports.models import Crashreport, HeartBeat
 
@@ -29,34 +32,35 @@
     """Filter reports matching a report counter requirements.
 
     Attributes:
-        model (django.db.model): The report model.
-        name (str): The human-readable report counter name.
-        field_name (str): The counter name as defined in the stats model where
-            it is a field.
+        model: The report model.
+        name: The human-readable report counter name.
+        field_name:
+            The counter name as defined in the stats model where it is a field.
 
     """
 
-    def __init__(self, model, name, field_name):
+    def __init__(self, model: Model, name: str, field_name: str) -> None:
         """Initialise the filter.
 
         Args:
-            model (django.db.model): The report model.
-            name (str): The human-readable report counter name.
-            field_name (str): The counter name as defined in the stats model
-                where it is a field.
+            model: The report model.
+            name: The human-readable report counter name.
+            field_name:
+                The counter name as defined in the stats model where it is a
+                field.
 
         """
         self.model = model
         self.name = name
         self.field_name = field_name
 
-    def filter(self, query_objects):
+    def filter(self, query_objects: QuerySet) -> QuerySet:
         """Filter the reports.
 
         Args:
-            query_objects (QuerySet): The reports to filter.
+            query_objects: The reports to filter.
         Returns:
-            QuerySet: The reports matching this report counter requirements.
+            The reports matching this report counter requirements.
 
         """
         # pylint: disable=no-self-use
@@ -78,38 +82,41 @@
     """The crashreports counter filter.
 
     Attributes:
-        include_boot_reasons (list(str)): The boot reasons assumed to
-            characterise this crashreport ("OR"ed).
-        exclude_boot_reasons (list(str)): The boot reasons assumed to *not*
-            characterise this crashreport ("AND"ed).
-        inclusive_filter (Q): The boot reasons filter for filtering reports
-            that should be included.
-        exclusive_filter (Q): The boot reasons filter for filtering reports
-            that should *not* be included.
+        include_boot_reasons:
+            The boot reasons assumed to characterise this crashreport ("OR"ed).
+        exclude_boot_reasons:
+            The boot reasons assumed to *not* characterise this crashreport (
+            "AND"ed).
+        inclusive_filter:
+            The boot reasons filter for filtering reports that should be
+            included.
+        exclusive_filter:
+            The boot reasons filter for filtering reports that should *not*
+            be included.
 
     """
 
     def __init__(
         self,
-        name,
-        field_name,
-        include_boot_reasons=None,
-        exclude_boot_reasons=None,
-    ):
+        name: str,
+        field_name: str,
+        include_boot_reasons: Optional[List[str]] = None,
+        exclude_boot_reasons: Optional[List[str]] = None,
+    ) -> None:
         """Initialise the filter.
 
         One or both of `include_boot_reasons` and `exclude_boot_reasons` must
         be specified.
 
         Args:
-            name (str): The human-readable report counter name.
-            field_name (str):
+            name: The human-readable report counter name.
+            field_name:
                 The counter name as defined in the stats model where it is a
                 field.
-            include_boot_reasons (list(str), optional):
+            include_boot_reasons:
                 The boot reasons assumed to characterise this crashreport
                 ("OR"ed).
-            exclude_boot_reasons (list(str), optional):
+            exclude_boot_reasons:
                 The boot reasons assumed to *not* characterise this
                 crashreport ("AND"ed).
         Raises:
@@ -137,14 +144,14 @@
         self.exclusive_filter = self._create_query_filter(exclude_boot_reasons)
 
     @staticmethod
-    def _create_query_filter(reasons):
+    def _create_query_filter(reasons: Optional[List[str]]) -> Q:
         """Combine boot reasons into one filter.
 
         Args:
-            reasons (list(str)): List of boot reasons to include in filter.
+            reasons: List of boot reasons to include in filter.
         Returns:
-            django.db.models.query_utils.Q: Query that matches either of
-                reasons as boot_reason if list is not empty, otherwise None.
+            Query that matches either of reasons as boot_reason if list is
+            not empty, otherwise None.
 
         """
         if not reasons:
@@ -155,13 +162,13 @@
             query = query | Q(boot_reason=reason)
         return query
 
-    def filter(self, query_objects):
+    def filter(self, query_objects: QuerySet) -> QuerySet:
         """Filter the reports according to the inclusive and exclusive fitlers.
 
         Args:
-            query_objects (QuerySet): The reports to filter.
+            query_objects: The reports to filter.
         Returns:
-            QuerySet: The reports matching this report counter requirements.
+            The reports matching this report counter requirements.
 
         """
         if self.inclusive_filter:
@@ -179,34 +186,42 @@
     counterparts (_DailyVersionStats).
     """
 
-    def __init__(self, stats_model, daily_stats_model, version_field_name):
+    def __init__(
+        self,
+        stats_model: _VersionStats,
+        daily_stats_model: _DailyVersionStats,
+        version_field_name: str,
+    ) -> None:
         """Initialise the engine.
 
         Args:
-            stats_model (_VersionStats): The _VersionStats model to update
-                stats for.
-            daily_stats_model (_DailyVersionStats): The _DailyVersionStats
-                model to update stats for.
-            version_field_name (str): The version field name as specified in
-                the stats models.
+            stats_model: The _VersionStats model to update stats for.
+            daily_stats_model: The _DailyVersionStats model to update stats for.
+            version_field_name:
+                The version field name as specified in the stats models.
 
         """
         self.stats_model = stats_model
         self.daily_stats_model = daily_stats_model
         self.version_field_name = version_field_name
 
-    def _valid_objects(self, query_objects):
+    def _valid_objects(self, query_objects: QuerySet) -> QuerySet:
         """Filter out invalid reports.
 
         Returns:
-            QuerySet: All the valid reports.
+            All the valid reports.
 
         """
         # pylint: disable=no-self-use
         # self is potentially used by subclasses.
         return query_objects
 
-    def _objects_within_period(self, query_objects, up_to, starting_from=None):
+    def _objects_within_period(
+        self,
+        query_objects: QuerySet,
+        up_to: datetime.datetime,
+        starting_from: Optional[datetime.datetime] = None,
+    ) -> QuerySet:
         """Retrieve the reports within a specific period of time.
 
         The objects are filtered considering a specific period of time to allow
@@ -215,12 +230,11 @@
         bound must be specified to avoid race conditions.
 
         Args:
-            query_objects (QuerySet): The reports to filter.
-            up_to (datetime): The maximum timestamp to consider (inclusive).
-            starting_from (datetime, optional): The minimum timestamp to
-                consider (exclusive).
+            query_objects: The reports to filter.
+            up_to: The maximum timestamp to consider (inclusive).
+            starting_from: The minimum timestamp to consider (exclusive).
         Returns:
-            QuerySet: The reports received within a specific period of time.
+            The reports received within a specific period of time.
 
         """
         # pylint: disable=no-self-use
@@ -231,13 +245,12 @@
 
         return query_objects
 
-    def _unique_objects_per_day(self, query_objects):
+    def _unique_objects_per_day(self, query_objects: QuerySet) -> QuerySet:
         """Count the unique reports per version per day.
 
         Args:
-            query_objects (QuerySet): The reports to count.
-        Returns:
-            QuerySet: The unique reports grouped per version per day.
+            query_objects: The reports to count.
+        Returns: The unique reports grouped per version per day.
 
         """
         return (
@@ -248,11 +261,10 @@
             .annotate(count=Count("date", distinct=True))
         )
 
-    def delete_stats(self):
+    def delete_stats(self) -> Dict[str, int]:
         """Delete the general and daily stats instances.
 
-        Returns:
-            dict(str, int): The count of deleted entries per model name.
+        Returns: The count of deleted entries per model name.
 
         """
         # Clear the general stats, the daily stats will be deleted by cascading
@@ -260,7 +272,12 @@
         _, count_per_model = self.stats_model.objects.all().delete()
         return count_per_model
 
-    def update_stats(self, report_counter, up_to, starting_from=None):
+    def update_stats(
+        self,
+        report_counter: _ReportCounterFilter,
+        up_to: datetime.datetime,
+        starting_from: Optional[datetime.datetime] = None,
+    ) -> Dict[Model, Dict[str, int]]:
         """Update the statistics of the general and daily stats entries.
 
         The algorithm works as follow:
@@ -275,15 +292,13 @@
             while the sum of them per version updates the general stats.
 
         Args:
-            report_counter (_ReportCounterEngine): The report counter to
-                update the stats with.
-            up_to (datetime): The maximum timestamp to consider (inclusive).
-            starting_from (datetime, optional): The minimum timestamp to
-                consider (exclusive).
+            report_counter: The report counter to update the stats with.
+            up_to: The maximum timestamp to consider (inclusive).
+            starting_from: The minimum timestamp to consider (exclusive).
         Returns:
-            dict(str, dict(str, int)): The number of added entries and the
-                number of updated entries bundled in a dict, respectively
-                hashed with the keys 'created' and 'updated', per model name.
+            The number of added entries and the number of updated entries
+            bundled in a dict, respectively hashed with the keys 'created'
+            and 'updated', per model name.
 
         """
         counts_per_model = {
diff --git a/crashreport_stats/models.py b/crashreport_stats/models.py
index e4bb683..461db80 100644
--- a/crashreport_stats/models.py
+++ b/crashreport_stats/models.py
@@ -1,71 +1,76 @@
 """The stats models."""
-from django.db import models
+from django.db.models import (
+    BooleanField,
+    CASCADE,
+    CharField,
+    DateField,
+    DateTimeField,
+    ForeignKey,
+    IntegerField,
+    Model,
+)
 
 
-class _VersionStats(models.Model):
+class _VersionStats(Model):
     """The base class for all-time stats of a version.
 
     Sub-classes should be created to gather stats about a versioned
     end-product such as a software build.
 
     Attributes:
-        is_official_release (models.BooleanField): If this version is an
-            official release. Defaults to False.
-        is_beta_release (models.BooleanField): If this version is a beta
-            release. Defaults to False.
-        first_seen_on (models.DateField): Day this version has been seen for
-            the first time as reported by devices (not by the server). Defaults
-            to the current date.
-        released_on (models.DateField): Day this version has been released on.
-            Defaults to the current date.
-        heartbeats (models.IntegerField): The total heartbeats counted for this
-            version.
-        prob_crashes (models.IntegerField): The total probable crash reports
-            counted for this version.
-        smpl (models.IntegerField): The total SMPL reports counted for this
-            version.
-        other (models.IntegerField): The total of other reports counted for
-            this version.
+        is_official_release:
+            If this version is an official release. Defaults to False.
+        is_beta_release: If this version is a beta release. Defaults to False.
+        first_seen_on:
+            Day this version has been seen for the first time as reported by
+            devices (not by the server). Defaults to the current date.
+        released_on:
+            Day this version has been released on. Defaults to the current date.
+        heartbeats: The total heartbeats counted for this version.
+        prob_crashes: The total probable crash reports counted for this version.
+        smpl: The total SMPL reports counted for this version.
+        other: The total of other reports counted for this version.
 
     """
 
-    is_official_release = models.BooleanField(default=False)
-    is_beta_release = models.BooleanField(default=False)
-    first_seen_on = models.DateField()
-    released_on = models.DateField()
-    heartbeats = models.IntegerField(default=0)
-    prob_crashes = models.IntegerField(default=0)
-    smpl = models.IntegerField(default=0)
-    other = models.IntegerField(default=0)
+    is_official_release = BooleanField(default=False)
+    is_beta_release = BooleanField(default=False)
+    first_seen_on = DateField()
+    released_on = DateField()
+    heartbeats = IntegerField(default=0)
+    prob_crashes = IntegerField(default=0)
+    smpl = IntegerField(default=0)
+    other = IntegerField(default=0)
 
     class Meta:
         abstract = True
 
 
-class _DailyVersionStats(models.Model):
+class _DailyVersionStats(Model):
     """The base class for daily stats of a version.
 
     Sub-classes MUST define the foreign key `version` pointing back to the
     `_VersionStats` implementation they are gathering stats for.
 
     Attributes:
-        date (models.DateField): Day considered for the stats.
-        heartbeats (models.IntegerField): The total heartbeats counted for this
-            version on the day `date`.
-        prob_crashes (models.IntegerField): The total probable crash reports
-            counted for this version on the day `date`.
-        smpl (models.IntegerField): The total SMPL reports counted for this
-            version on the day `date`.
-        other (models.IntegerField): The total of other reports counted for
-            this version on the day `date`.
+        date: Day considered for the stats.
+        heartbeats:
+            The total heartbeats counted for this version on the day `date`.
+        prob_crashes:
+            The total probable crash reports counted for this version on the
+            day `date`.
+        smpl: The total SMPL reports counted for this version on the day `date`.
+        other:
+            The total of other reports counted for this version on the day
+            `date`.
 
     """
 
-    date = models.DateField()
-    heartbeats = models.IntegerField(default=0)
-    prob_crashes = models.IntegerField(default=0)
-    smpl = models.IntegerField(default=0)
-    other = models.IntegerField(default=0)
+    date = DateField()
+    heartbeats = IntegerField(default=0)
+    prob_crashes = IntegerField(default=0)
+    smpl = IntegerField(default=0)
+    other = IntegerField(default=0)
 
     class Meta:
         abstract = True
@@ -75,12 +80,12 @@
     """The all-time stats of a software version.
 
     Attributes:
-        build_fingerprint (models.CharField): The software build fingerprint
-            uniquely identifying this version.
+        build_fingerprint:
+            The software build fingerprint uniquely identifying this version.
 
     """
 
-    build_fingerprint = models.CharField(max_length=200, unique=True)
+    build_fingerprint = CharField(max_length=200, unique=True)
 
     def __str__(self):  # noqa: D105
         return self.build_fingerprint
@@ -90,16 +95,13 @@
     """The daily stats of a software version.
 
     Attributes:
-        version (models.ForeignKey): The software version object (`Version`)
-            these daily stats are about.
+        version:
+            The software version object (`Version`) these daily stats are about.
 
     """
 
-    version = models.ForeignKey(
-        Version,
-        db_index=True,
-        related_name="daily_stats",
-        on_delete=models.CASCADE,
+    version = ForeignKey(
+        Version, db_index=True, related_name="daily_stats", on_delete=CASCADE
     )
 
 
@@ -107,12 +109,12 @@
     """The all-time stats of a radio version.
 
     Attributes:
-        radio_version (models.CharField): The radio version number uniquely
-            identifying this version.
+        radio_version:
+            The radio version number uniquely identifying this version.
 
     """
 
-    radio_version = models.CharField(max_length=200, unique=True)
+    radio_version = CharField(max_length=200, unique=True)
 
     def __str__(self):  # noqa: D105
         return self.radio_version
@@ -122,26 +124,26 @@
     """The daily stats of a radio version.
 
     Attributes:
-        version (models.ForeignKey): The radio version object (`RadioVersion`)
-            these daily stats are about.
+        version:
+            The radio version object (`RadioVersion`) these daily stats are
+            about.
 
     """
 
-    version = models.ForeignKey(
+    version = ForeignKey(
         RadioVersion,
         db_index=True,
         related_name="daily_stats",
-        on_delete=models.CASCADE,
+        on_delete=CASCADE,
     )
 
 
-class StatsMetadata(models.Model):
+class StatsMetadata(Model):
     """The stats metadata.
 
     Attributes:
-        updated_at (models.DateTimeField): The last time the stats were
-            updated.
+        updated_at: The last time the stats were updated.
 
     """
 
-    updated_at = models.DateTimeField()
+    updated_at = DateTimeField()
diff --git a/crashreports/response_descriptions.py b/crashreports/response_descriptions.py
index 714f513..f97ccdf 100644
--- a/crashreports/response_descriptions.py
+++ b/crashreports/response_descriptions.py
@@ -1,18 +1,20 @@
 """Response descriptions for HTTP responses."""
+from typing import Tuple, Type
+
+from rest_framework.exceptions import APIException
 
 
-def default_desc(exception):
+def default_desc(exception: Type[APIException]) -> Tuple[int, str]:
     """Get the default response description for an exception.
 
     Args:
-        exception (rest_framework.exceptions.APIException):
+        exception:
             A subclass of APIException for which the response description
             should be returned.
 
     Returns:
-        (int, str):
-            A tuple containing the matching status code and default description
-            for the exception.
+        A tuple containing the matching status code and default description
+        for the exception.
 
     """
     return exception.status_code, str(exception.default_detail)
diff --git a/crashreports/tests.py b/crashreports/tests.py
index 354b330..b70f400 100644
--- a/crashreports/tests.py
+++ b/crashreports/tests.py
@@ -1,6 +1,7 @@
 """Test the API for crashreports, devices, heartbeats and logfiles."""
 import os
 import tempfile
+from typing import Optional
 
 from django.contrib.auth.models import User
 from django.urls import reverse
@@ -101,14 +102,14 @@
         return Dummy._update_copy(Dummy.DEFAULT_DUMMY_HEARTBEAT_VALUES, kwargs)
 
     @staticmethod
-    def crashreport_data(report_type=None, **kwargs):
+    def crashreport_data(report_type: Optional[str] = None, **kwargs):
         """Return the data required to create a crashreport.
 
         Use the values passed as keyword arguments or default to the ones
         from `Dummy.DEFAULT_DUMMY_CRASHREPORTS_VALUES`.
 
         Args:
-            report_type (str, optional): A valid value from
+            report_type: A valid value from
                 `Dummy.CRASH_TYPE_TO_BOOT_REASON_MAP.keys()` that will
                 define the boot reason if not explicitly defined in the
                 keyword arguments already.