diff --git a/src/icalendar/attr.py b/src/icalendar/attr.py
index 545f66c48..2b15d6b07 100644
--- a/src/icalendar/attr.py
+++ b/src/icalendar/attr.py
@@ -989,7 +989,7 @@ def property_get_duration(self: Component) -> timedelta | None:
     return result
 
 
-def property_set_duration(self: Component, value: timedelta | None):
+def property_set_duration(self: Component, value: timedelta | None) -> None:
     """Setter for property DURATION."""
     if value is None:
         self.pop("duration", None)
@@ -1001,7 +1001,7 @@ def property_set_duration(self: Component, value: timedelta | None):
     self.pop("DUE")
 
 
-def property_del_duration(self: Component):
+def property_del_duration(self: Component) -> None:
     """Delete property DURATION."""
     self.pop("DURATION")
 
@@ -1498,7 +1498,7 @@ def fset(self: Component, value: str | StrEnum | None) -> None:
 )
 
 
-def timezone_datetime_property(name: str, docs: str):
+def timezone_datetime_property(name: str, docs: str) -> property:
     """Create a property to access the values with a proper timezone."""
 
     return single_utc_property(name, docs)
@@ -1583,12 +1583,12 @@ def rfc_7953_end_property(self) -> timedelta | None:
 
 
 @rfc_7953_end_property.setter
-def rfc_7953_end_property(self, value: datetime):
+def rfc_7953_end_property(self, value: datetime) -> None:
     self.DTEND = value
 
 
 @rfc_7953_end_property.deleter
-def rfc_7953_end_property(self):
+def rfc_7953_end_property(self) -> None:
     del self.DTEND
 
 
@@ -2376,7 +2376,7 @@ def _del_concepts(self: Component):
 concepts_property = property(_get_concepts, _set_concepts, _del_concepts)
 
 
-def multi_string_property(name: str, doc: str):
+def multi_string_property(name: str, doc: str) -> property:
     """A property for an iCalendar Property that can occur multiple times."""
 
     def fget(self: Component) -> list[str]:
diff --git a/src/icalendar/cli.py b/src/icalendar/cli.py
index 255757004..e3d444a7c 100755
--- a/src/icalendar/cli.py
+++ b/src/icalendar/cli.py
@@ -86,7 +86,7 @@ def view(event: Event) -> str:
 {description}"""
 
 
-def main():
+def main() -> None:
     parser = argparse.ArgumentParser(description=__doc__)
 
     parser.add_argument(
diff --git a/src/icalendar/fuzzing/ical_fuzzer.py b/src/icalendar/fuzzing/ical_fuzzer.py
index a2d776099..63ceb6773 100755
--- a/src/icalendar/fuzzing/ical_fuzzer.py
+++ b/src/icalendar/fuzzing/ical_fuzzer.py
@@ -50,7 +50,7 @@ def TestOneInput(data):
     print("--- end calendar ---")
 
 
-def main():
+def main() -> None:
     atheris.Setup(sys.argv, TestOneInput)
     atheris.Fuzz()
 
diff --git a/src/icalendar/parser/parameter.py b/src/icalendar/parser/parameter.py
index 08a135231..e3517d4da 100644
--- a/src/icalendar/parser/parameter.py
+++ b/src/icalendar/parser/parameter.py
@@ -189,7 +189,7 @@ def q_join(lst: Sequence[str], sep: str = ",", always_quote: bool = False) -> st
     return sep.join(dquote(itm, always_quote=always_quote) for itm in lst)
 
 
-def single_string_parameter(func: Callable | None = None, upper=False):
+def single_string_parameter(func: Callable | None = None, upper=False) -> Callable | property:
     """Create a parameter getter/setter for a single string parameter.
 
     Parameters:
diff --git a/src/icalendar/parser/property.py b/src/icalendar/parser/property.py
index 6efca840a..91f12bb80 100644
--- a/src/icalendar/parser/property.py
+++ b/src/icalendar/parser/property.py
@@ -25,7 +25,7 @@ def unescape_list_or_string(val: str | list[str]) -> str | list[str]:
 _unescape_backslash_regex = re.compile(r"\\([\\,;:nN])")
 
 
-def unescape_backslash(val: str):
+def unescape_backslash(val: str) -> str:
     r"""Unescape backslash sequences in iCalendar text.
 
     Unlike :py:meth:`unescape_string`, this only handles actual backslash escapes
diff --git a/src/icalendar/timezone/__init__.py b/src/icalendar/timezone/__init__.py
index 0c94cbd49..f8168232d 100644
--- a/src/icalendar/timezone/__init__.py
+++ b/src/icalendar/timezone/__init__.py
@@ -6,12 +6,12 @@
 tzp = TZP()
 
 
-def use_pytz():
+def use_pytz() -> None:
     """Use pytz as the implementation that looks up and creates timezones."""
     tzp.use_pytz()
 
 
-def use_zoneinfo():
+def use_zoneinfo() -> None:
     """Use zoneinfo as the implementation that looks up and creates timezones."""
     tzp.use_zoneinfo()
 
diff --git a/src/icalendar/timezone/zoneinfo.py b/src/icalendar/timezone/zoneinfo.py
index 24733a4cc..72f8cb820 100644
--- a/src/icalendar/timezone/zoneinfo.py
+++ b/src/icalendar/timezone/zoneinfo.py
@@ -120,7 +120,7 @@ def uses_zoneinfo(self) -> bool:
         return True
 
 
-def pickle_tzicalvtz(tzicalvtz: _tzicalvtz):
+def pickle_tzicalvtz(tzicalvtz: _tzicalvtz) -> tuple[type[_tzicalvtz], tuple]:
     """Because we use dateutil.tzical, we need to make it pickle-able."""
     return _tzicalvtz, (tzicalvtz._tzid, tzicalvtz._comps)  # noqa: SLF001
 
@@ -128,7 +128,7 @@ def pickle_tzicalvtz(tzicalvtz: _tzicalvtz):
 copyreg.pickle(_tzicalvtz, pickle_tzicalvtz)
 
 
-def pickle_rrule_with_cache(self: rrule):
+def pickle_rrule_with_cache(self: rrule) -> tuple[functools.partial, tuple]:
     """Make sure we can also pickle rrules that cache.
 
     This is mainly copied from rrule.replace.
@@ -150,7 +150,7 @@ def pickle_rrule_with_cache(self: rrule):
 copyreg.pickle(rrule, pickle_rrule_with_cache)
 
 
-def pickle_rruleset_with_cache(rs: rruleset):
+def pickle_rruleset_with_cache(rs: rruleset) -> tuple[type[unpickle_rruleset_with_cache], tuple]:
     """Pickle an rruleset."""
     return unpickle_rruleset_with_cache, (
         rs._rrule,  # noqa: SLF001
@@ -161,7 +161,7 @@ def pickle_rruleset_with_cache(rs: rruleset):
     )
 
 
-def unpickle_rruleset_with_cache(rrule, rdate, exrule, exdate, cache):
+def unpickle_rruleset_with_cache(rrule, rdate, exrule, exdate, cache) -> rruleset:
     """unpickling the rruleset."""
     rs = rruleset(cache)
     for o in rrule: