diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4b0f3e63a..ade861023 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Changelog
 
+### 41.5.5 [#1220](https://github.com/openfisca/openfisca-core/pull/1220)
+
+#### Technical changes
+
+- Fix doc & type definitions in the entities module
+
 ### 41.5.4 [#1219](https://github.com/openfisca/openfisca-core/pull/1219)
 
 #### Technical changes
diff --git a/openfisca_core/entities/__init__.py b/openfisca_core/entities/__init__.py
index a1cd397a3..9546773cb 100644
--- a/openfisca_core/entities/__init__.py
+++ b/openfisca_core/entities/__init__.py
@@ -27,4 +27,14 @@
 from .helpers import build_entity, find_role
 from .role import Role
 
-__all__ = ["Entity", "GroupEntity", "Role", "build_entity", "find_role", "types"]
+SingleEntity = Entity
+
+__all__ = [
+    "Entity",
+    "SingleEntity",
+    "GroupEntity",
+    "Role",
+    "build_entity",
+    "find_role",
+    "types",
+]
diff --git a/openfisca_core/entities/_core_entity.py b/openfisca_core/entities/_core_entity.py
index ecd17beca..9a2707d19 100644
--- a/openfisca_core/entities/_core_entity.py
+++ b/openfisca_core/entities/_core_entity.py
@@ -1,24 +1,19 @@
 from __future__ import annotations
 
-from typing import Any
-
-from openfisca_core.types import TaxBenefitSystem, Variable
-
 import os
 from abc import abstractmethod
 
+from . import types as t
 from .role import Role
-from .types import Entity
 
 
 class _CoreEntity:
     """Base class to build entities from."""
 
     #: A key to identify the entity.
-    key: str
-
+    key: t.EntityKey
     #: The ``key``, pluralised.
-    plural: str | None
+    plural: t.EntityPlural | None
 
     #: A summary description.
     label: str | None
@@ -30,16 +25,18 @@ class _CoreEntity:
     is_person: bool
 
     #: A TaxBenefitSystem instance.
-    _tax_benefit_system: TaxBenefitSystem | None = None
+    _tax_benefit_system: t.TaxBenefitSystem | None = None
 
     @abstractmethod
-    def __init__(self, key: str, plural: str, label: str, doc: str, *args: Any) -> None:
+    def __init__(
+        self, key: str, plural: str, label: str, doc: str, *args: object
+    ) -> None:
         ...
 
     def __repr__(self) -> str:
         return f"{self.__class__.__name__}({self.key})"
 
-    def set_tax_benefit_system(self, tax_benefit_system: TaxBenefitSystem) -> None:
+    def set_tax_benefit_system(self, tax_benefit_system: t.TaxBenefitSystem) -> None:
         """An Entity belongs to a TaxBenefitSystem."""
         self._tax_benefit_system = tax_benefit_system
 
@@ -47,14 +44,18 @@ def get_variable(
         self,
         variable_name: str,
         check_existence: bool = False,
-    ) -> Variable | None:
+    ) -> t.Variable | None:
         """Get a ``variable_name`` from ``variables``."""
+        if self._tax_benefit_system is None:
+            raise ValueError(
+                "You must set 'tax_benefit_system' before calling this method."
+            )
         return self._tax_benefit_system.get_variable(variable_name, check_existence)
 
     def check_variable_defined_for_entity(self, variable_name: str) -> None:
         """Check if ``variable_name`` is defined for ``self``."""
-        variable: Variable | None
-        entity: Entity
+        variable: t.Variable | None
+        entity: t.CoreEntity
 
         variable = self.get_variable(variable_name, check_existence=True)
 
@@ -71,7 +72,7 @@ def check_variable_defined_for_entity(self, variable_name: str) -> None:
             )
             raise ValueError(os.linesep.join(message))
 
-    def check_role_validity(self, role: Any) -> None:
+    def check_role_validity(self, role: object) -> None:
         """Check if a ``role`` is an instance of Role."""
         if role is not None and not isinstance(role, Role):
             raise ValueError(f"{role} is not a valid role")
diff --git a/openfisca_core/entities/entity.py b/openfisca_core/entities/entity.py
index ea2deb505..819477266 100644
--- a/openfisca_core/entities/entity.py
+++ b/openfisca_core/entities/entity.py
@@ -1,5 +1,6 @@
 import textwrap
 
+from . import types as t
 from ._core_entity import _CoreEntity
 
 
@@ -9,8 +10,8 @@ class Entity(_CoreEntity):
     """
 
     def __init__(self, key: str, plural: str, label: str, doc: str) -> None:
-        self.key = key
+        self.key = t.EntityKey(key)
         self.label = label
-        self.plural = plural
+        self.plural = t.EntityPlural(plural)
         self.doc = textwrap.dedent(doc)
         self.is_person = True
diff --git a/openfisca_core/entities/group_entity.py b/openfisca_core/entities/group_entity.py
index eeae52d38..d2242983d 100644
--- a/openfisca_core/entities/group_entity.py
+++ b/openfisca_core/entities/group_entity.py
@@ -1,11 +1,11 @@
 from __future__ import annotations
 
-from collections.abc import Iterable, Mapping
-from typing import Any
+from collections.abc import Iterable, Sequence
 
 import textwrap
 from itertools import chain
 
+from . import types as t
 from ._core_entity import _CoreEntity
 from .role import Role
 
@@ -34,12 +34,12 @@ def __init__(
         plural: str,
         label: str,
         doc: str,
-        roles: Iterable[Mapping[str, Any]],
+        roles: Sequence[t.RoleParams],
         containing_entities: Iterable[str] = (),
     ) -> None:
-        self.key = key
+        self.key = t.EntityKey(key)
         self.label = label
-        self.plural = plural
+        self.plural = t.EntityPlural(plural)
         self.doc = textwrap.dedent(doc)
         self.is_person = False
         self.roles_description = roles
diff --git a/openfisca_core/entities/helpers.py b/openfisca_core/entities/helpers.py
index 000c8028c..90b9ffd94 100644
--- a/openfisca_core/entities/helpers.py
+++ b/openfisca_core/entities/helpers.py
@@ -1,22 +1,23 @@
 from __future__ import annotations
 
-from typing import Iterable
+from collections.abc import Iterable, Sequence
+from typing import Optional
 
+from . import types as t
 from .entity import Entity
 from .group_entity import GroupEntity
-from .types import Role
 
 
 def build_entity(
-    key,
-    plural,
-    label,
-    doc="",
-    roles=None,
-    is_person=False,
-    class_override=None,
-    containing_entities=(),
-):
+    key: str,
+    plural: str,
+    label: str,
+    doc: str = "",
+    roles: Optional[Sequence[t.RoleParams]] = None,
+    is_person: bool = False,
+    class_override: object | None = None,
+    containing_entities: Sequence[str] = (),
+) -> t.SingleEntity | t.GroupEntity:
     """Build a SingleEntity or a GroupEntity.
 
     Args:
@@ -35,7 +36,7 @@ def build_entity(
         :obj:`.GroupEntity`: When ``is_person`` is False.
 
     Raises:
-        ValueError if ``roles`` is not a sequence.
+        NotImplementedError: if ``roles`` is None.
 
     Examples:
         >>> from openfisca_core import entities
@@ -73,15 +74,17 @@ def build_entity(
     if is_person:
         return Entity(key, plural, label, doc)
 
-    else:
+    if roles is not None:
         return GroupEntity(
             key, plural, label, doc, roles, containing_entities=containing_entities
         )
 
+    raise NotImplementedError
+
 
 def find_role(
-    roles: Iterable[Role], key: str, *, total: int | None = None
-) -> Role | None:
+    roles: Iterable[t.Role], key: t.RoleKey, *, total: int | None = None
+) -> t.Role | None:
     """Find a Role in a GroupEntity.
 
     Args:
diff --git a/openfisca_core/entities/role.py b/openfisca_core/entities/role.py
index a4cb75a86..d70357816 100644
--- a/openfisca_core/entities/role.py
+++ b/openfisca_core/entities/role.py
@@ -6,7 +6,7 @@
 import dataclasses
 import textwrap
 
-from .types import Entity
+from .types import SingleEntity
 
 
 class Role:
@@ -48,7 +48,7 @@ class Role:
     """
 
     #: The Entity the Role belongs to.
-    entity: Entity
+    entity: SingleEntity
 
     #: A description of the Role.
     description: _Description
@@ -79,7 +79,7 @@ def doc(self) -> str | None:
         """A full description, non-indented."""
         return self.description.doc
 
-    def __init__(self, description: Mapping[str, Any], entity: Entity) -> None:
+    def __init__(self, description: Mapping[str, Any], entity: SingleEntity) -> None:
         self.description = _Description(
             **{
                 key: value
diff --git a/openfisca_core/entities/types.py b/openfisca_core/entities/types.py
index 1a2fb06b2..2f9acd040 100644
--- a/openfisca_core/entities/types.py
+++ b/openfisca_core/entities/types.py
@@ -1,30 +1,61 @@
 from __future__ import annotations
 
 from collections.abc import Iterable
-from typing import Protocol, TypedDict
+from typing import NewType, Protocol
+from typing_extensions import Required, TypedDict
 
+from openfisca_core import types as t
 
-class Entity(Protocol):
+# Entities
+
+#: For example "person".
+EntityKey = NewType("EntityKey", str)
+
+#: For example "persons".
+EntityPlural = NewType("EntityPlural", str)
+
+#: For example "principal".
+RoleKey = NewType("RoleKey", str)
+
+#: For example "parents".
+RolePlural = NewType("RolePlural", str)
+
+
+class CoreEntity(t.CoreEntity, Protocol):
+    key: EntityKey
+    plural: EntityPlural | None
+
+
+class SingleEntity(t.SingleEntity, Protocol):
     ...
 
 
-class GroupEntity(Protocol):
+class GroupEntity(t.GroupEntity, Protocol):
     ...
 
 
-class Role(Protocol):
-    max: int | None
+class Role(t.Role, Protocol):
     subroles: Iterable[Role] | None
 
-    @property
-    def key(self) -> str:
-        ...
-
 
 class RoleParams(TypedDict, total=False):
-    key: str
+    key: Required[str]
     plural: str
     label: str
     doc: str
     max: int
     subroles: list[str]
+
+
+# Tax-Benefit systems
+
+
+class TaxBenefitSystem(t.TaxBenefitSystem, Protocol):
+    ...
+
+
+# Variables
+
+
+class Variable(t.Variable, Protocol):
+    ...
diff --git a/openfisca_core/populations/population.py b/openfisca_core/populations/population.py
index 06d680388..e3ef6b209 100644
--- a/openfisca_core/populations/population.py
+++ b/openfisca_core/populations/population.py
@@ -3,7 +3,7 @@
 from typing import Dict, NamedTuple, Optional, Sequence, Union
 from typing_extensions import TypedDict
 
-from openfisca_core.types import Array, Entity, Period, Role, Simulation
+from openfisca_core.types import Array, Period, Role, Simulation, SingleEntity
 
 import traceback
 
@@ -16,12 +16,12 @@
 
 class Population:
     simulation: Optional[Simulation]
-    entity: Entity
+    entity: SingleEntity
     _holders: Dict[str, holders.Holder]
     count: int
     ids: Array[str]
 
-    def __init__(self, entity: Entity) -> None:
+    def __init__(self, entity: SingleEntity) -> None:
         self.simulation = None
         self.entity = entity
         self._holders = {}
diff --git a/openfisca_core/projectors/helpers.py b/openfisca_core/projectors/helpers.py
index ce94f9773..b3b7e6f2d 100644
--- a/openfisca_core/projectors/helpers.py
+++ b/openfisca_core/projectors/helpers.py
@@ -2,7 +2,7 @@
 
 from collections.abc import Mapping
 
-from openfisca_core.entities.types import Entity, GroupEntity, Role
+from openfisca_core.types import GroupEntity, Role, SingleEntity
 
 from openfisca_core import entities, projectors
 
@@ -110,7 +110,7 @@ def get_projector_from_shortcut(
 
     """
 
-    entity: Entity | GroupEntity = population.entity
+    entity: SingleEntity | GroupEntity = population.entity
 
     if isinstance(entity, entities.Entity):
         populations: Mapping[
diff --git a/openfisca_core/projectors/typing.py b/openfisca_core/projectors/typing.py
index a6ce8e398..186f90e30 100644
--- a/openfisca_core/projectors/typing.py
+++ b/openfisca_core/projectors/typing.py
@@ -3,12 +3,12 @@
 from collections.abc import Mapping
 from typing import Protocol
 
-from openfisca_core.entities.types import Entity, GroupEntity
+from openfisca_core.types import GroupEntity, SingleEntity
 
 
 class Population(Protocol):
     @property
-    def entity(self) -> Entity:
+    def entity(self) -> SingleEntity:
         ...
 
     @property
diff --git a/openfisca_core/types.py b/openfisca_core/types.py
index ede4ba9bc..b34a55543 100644
--- a/openfisca_core/types.py
+++ b/openfisca_core/types.py
@@ -30,7 +30,7 @@
 # Entities
 
 
-class Entity(Protocol):
+class CoreEntity(Protocol):
     key: Any
     plural: Any
 
@@ -51,10 +51,23 @@ def get_variable(
         ...
 
 
+class SingleEntity(CoreEntity, Protocol):
+    ...
+
+
+class GroupEntity(CoreEntity, Protocol):
+    ...
+
+
 class Role(Protocol):
     entity: Any
+    max: int | None
     subroles: Any
 
+    @property
+    def key(self) -> str:
+        ...
+
 
 # Holders
 
diff --git a/openfisca_tasks/lint.mk b/openfisca_tasks/lint.mk
index 828c51800..445abba10 100644
--- a/openfisca_tasks/lint.mk
+++ b/openfisca_tasks/lint.mk
@@ -41,6 +41,7 @@ check-types:
 	@$(call print_help,$@:)
 	@mypy \
 		openfisca_core/commons \
+		openfisca_core/entities \
 		openfisca_core/types.py
 	@$(call print_pass,$@:)
 
diff --git a/setup.cfg b/setup.cfg
index e266be625..cc850c06a 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -1,85 +1,86 @@
-# C011X:            We (progressively) document the code base.
-# D10X:             We (progressively) check docstrings (see https://www.pydocstyle.org/en/2.1.1/error_codes.html#grouping).
-# DARXXX:           We (progressively) check docstrings (see https://github.com/terrencepreilly/darglint#error-codes).
-# E203:             We ignore a false positive in whitespace before ":" (see https://github.com/PyCQA/pycodestyle/issues/373).
-# F403/405:         We ignore * imports.
-# R0401:            We avoid cyclic imports —required for unit/doc tests.
-# RST301:           We use Google Python Style (see https://pypi.org/project/flake8-rst-docstrings/).
-# W503/504:         We break lines before binary operators (Knuth's style).
+# C011X:                    We (progressively) document the code base.
+# D10X:                     We (progressively) check docstrings (see https://www.pydocstyle.org/en/2.1.1/error_codes.html#grouping).
+# DARXXX:                   We (progressively) check docstrings (see https://github.com/terrencepreilly/darglint#error-codes).
+# E203:                     We ignore a false positive in whitespace before ":" (see https://github.com/PyCQA/pycodestyle/issues/373).
+# F403/405:                 We ignore * imports.
+# R0401:                    We avoid cyclic imports —required for unit/doc tests.
+# RST301:                   We use Google Python Style (see https://pypi.org/project/flake8-rst-docstrings/).
+# W503/504:                 We break lines before binary operators (Knuth's style).
 
 [flake8]
-convention          = google
-docstring_style     = google
-extend-ignore       = D
-ignore              = E203, E501, F405, RST301, W503
-in-place            = true
-include-in-doctest  = openfisca_core/commons openfisca_core/entities openfisca_core/holders openfisca_core/periods openfisca_core/projectors
-max-line-length     = 88
-per-file-ignores    = */types.py:D101,D102,E704, */test_*.py:D101,D102,D103, */__init__.py:F401
-rst-directives      = attribute, deprecated, seealso, versionadded, versionchanged
-rst-roles           = any, attr, class, exc, func, meth, mod, obj
-strictness          = short
+convention                  = google
+docstring_style             = google
+extend-ignore               = D
+ignore                      = E203, E501, F405, RST301, W503
+in-place                    = true
+include-in-doctest          = openfisca_core/commons openfisca_core/entities openfisca_core/holders openfisca_core/periods openfisca_core/projectors
+max-line-length             = 88
+per-file-ignores            = */types.py:D101,D102,E704, */test_*.py:D101,D102,D103, */__init__.py:F401
+rst-directives              = attribute, deprecated, seealso, versionadded, versionchanged
+rst-roles                   = any, attr, class, exc, func, meth, mod, obj
+strictness                  = short
 
 [pylint.MASTER]
-load-plugins        = pylint_per_file_ignores
+load-plugins                = pylint_per_file_ignores
 
 [pylint.message_control]
-disable             = all
-enable              = C0115, C0116, R0401
-per-file-ignores    =
+disable                     = all
+enable                      = C0115, C0116, R0401
+per-file-ignores            =
     types.py:C0115,C0116
     /tests/:C0116
-score               = no
+score                       = no
 
 [isort]
-case_sensitive      = true
+case_sensitive              = true
 force_alphabetical_sort_within_sections = false
-group_by_package    = true
-honor_noqa          = true
-include_trailing_comma = true
-known_first_party   = openfisca_core
-known_openfisca     = openfisca_country_template, openfisca_extension_template
-known_typing        = *collections.abc*, *typing*, *typing_extensions*
-known_types         = *types*
-profile             = black
-py_version          = 39
-sections            = FUTURE, TYPING, TYPES, STDLIB, THIRDPARTY, OPENFISCA, FIRSTPARTY, LOCALFOLDER
+group_by_package            = true
+honor_noqa                  = true
+include_trailing_comma      = true
+known_first_party           = openfisca_core
+known_openfisca             = openfisca_country_template, openfisca_extension_template
+known_typing                = *collections.abc*, *typing*, *typing_extensions*
+known_types                 = *types*
+profile                     = black
+py_version                  = 39
+sections                    = FUTURE, TYPING, TYPES, STDLIB, THIRDPARTY, OPENFISCA, FIRSTPARTY, LOCALFOLDER
 
 [coverage:paths]
-source              = . */site-packages
+source                      = . */site-packages
 
 [coverage:run]
-branch              = true
-source              = openfisca_core, openfisca_web_api
+branch                      = true
+source                      = openfisca_core, openfisca_web_api
 
 [coverage:report]
-fail_under          = 75
-show_missing        = true
-skip_covered        = true
-skip_empty          = true
+fail_under                  = 75
+show_missing                = true
+skip_covered                = true
+skip_empty                  = true
 
 [tool:pytest]
-addopts             = --doctest-modules --disable-pytest-warnings --showlocals
-doctest_optionflags = ELLIPSIS IGNORE_EXCEPTION_DETAIL NUMBER NORMALIZE_WHITESPACE
-python_files        = **/*.py
-testpaths           = tests
+addopts                     = --disable-pytest-warnings --doctest-modules --showlocals
+doctest_optionflags         = ELLIPSIS IGNORE_EXCEPTION_DETAIL NUMBER NORMALIZE_WHITESPACE
+python_files                = **/*.py
+testpaths                   = tests
 
 [mypy]
-disallow_any_unimported = true
-ignore_missing_imports = true
-install_types       = true
-non_interactive     = true
-plugins             = numpy.typing.mypy_plugin
-python_version      = 3.9
+check_untyped_defs          = false
+disallow_any_decorated      = false
+disallow_any_explicit       = false
+disallow_any_expr           = false
+disallow_any_unimported     = false
+follow_imports              = skip
+ignore_missing_imports      = true
+implicit_reexport           = false
+install_types               = true
+non_interactive             = true
+plugins                     = numpy.typing.mypy_plugin
+pretty                      = true
+python_version              = 3.9
+strict                      = false
+warn_no_return              = true
+warn_unreachable            = true
 
-[mypy-openfisca_core.commons.tests.*]
-ignore_errors       = True
-
-[mypy-openfisca_core.holders.tests.*]
-ignore_errors       = True
-
-[mypy-openfisca_core.periods.tests.*]
-ignore_errors 		= True
-
-[mypy-openfisca_core.scripts.*]
-ignore_errors       = True
+[mypy-openfisca_core.*.tests.*]
+ignore_errors               = True
diff --git a/setup.py b/setup.py
index 92971f40b..cca107bee 100644
--- a/setup.py
+++ b/setup.py
@@ -67,7 +67,7 @@
 
 setup(
     name="OpenFisca-Core",
-    version="41.5.4",
+    version="41.5.5",
     author="OpenFisca Team",
     author_email="contact@openfisca.org",
     classifiers=[