diff --git a/openhtf/core/base_plugs.py b/openhtf/core/base_plugs.py
index 6fddc73e..819a3467 100644
--- a/openhtf/core/base_plugs.py
+++ b/openhtf/core/base_plugs.py
@@ -128,17 +128,17 @@ def tearDown(self):
       via TestApi.
   """
   # Override this to True in subclasses to support remote Plug access.
-  enable_remote = False  # type: bool
+  enable_remote: bool = False
   # Allow explicitly disabling remote access to specific attributes.
-  disable_remote_attrs = set()  # type: Set[Text]
+  disable_remote_attrs = set()
   # Override this to True in subclasses to support using with_plugs with this
   # plug without needing to use placeholder.  This will only affect the classes
   # that explicitly define this; subclasses do not share the declaration.
-  auto_placeholder = False  # type: bool
+  auto_placeholder: bool = False
   # Default logger to be used only in __init__ of subclasses.
   # This is overwritten both on the class and the instance so don't store
   # a copy of it anywhere.
-  logger = _LOG  # type: logging.Logger
+  logger: logging.Logger = _LOG
 
   @util.classproperty
   def placeholder(cls) -> 'PlugPlaceholder':  # pylint: disable=no-self-argument
@@ -185,7 +185,7 @@ class FrontendAwareBasePlug(BasePlug, util.SubscribableStateMixin):
   Since the Station API runs in a separate thread, the _asdict() method of
   frontend-aware plugs should be written with thread safety in mind.
   """
-  enable_remote = True  # type: bool
+  enable_remote: bool = True
 
 
 @attr.s(slots=True, frozen=True)
diff --git a/openhtf/core/phase_collections.py b/openhtf/core/phase_collections.py
index 9772e574..0e0425a2 100644
--- a/openhtf/core/phase_collections.py
+++ b/openhtf/core/phase_collections.py
@@ -221,16 +221,18 @@ def check_for_duplicate_subtest_names(sequence: PhaseSequence):
   Raises:
     DuplicateSubtestNamesError: when duplicate subtest names are found.
   """
-  names_to_subtests = collections.defaultdict(
-      list)  # type: DefaultDict[Text, List[Subtest]]
+  names_to_subtests: collections.defaultdict[str, list[Subtest]] = (
+      collections.defaultdict(list)
+  )
   for subtest in sequence.filter_by_type(Subtest):
     names_to_subtests[subtest.name].append(subtest)
 
-  duplicates = []  # type: List[Text]
+  duplicates: list[str] = []
   for name, subtests in names_to_subtests.items():
     if len(subtests) > 1:
-      duplicates.append('Name "{}" used by multiple subtests: {}'.format(
-          name, subtests))
+      duplicates.append(
+          'Name "{}" used by multiple subtests: {}'.format(name, subtests)
+      )
   if not duplicates:
     return
   duplicates.sort()
diff --git a/openhtf/output/callbacks/__init__.py b/openhtf/output/callbacks/__init__.py
index 20e5eaa0..5463c13c 100644
--- a/openhtf/output/callbacks/__init__.py
+++ b/openhtf/output/callbacks/__init__.py
@@ -25,7 +25,7 @@
 import shutil
 import tempfile
 import typing
-from typing import BinaryIO, Callable, Iterator, Text, Union
+from typing import BinaryIO, Callable, Iterator, Optional, Text, Union
 
 from openhtf import util
 from openhtf.core import test_record
@@ -80,8 +80,8 @@ class OutputToFile(object):
 
   def __init__(self, filename_pattern_or_file: Union[Text, Callable[..., Text],
                                                      BinaryIO]):
-    self.filename_pattern = None  # type: Optional[Union[Text, Callable[..., Text]]]
-    self.output_file = None  # type: Optional[BinaryIO]
+    self.filename_pattern: Optional[Union[Text, Callable[..., Text]]] = None
+    self.output_file: Optional[BinaryIO] = None
     if (isinstance(filename_pattern_or_file, str) or
         callable(filename_pattern_or_file)):
       self.filename_pattern = filename_pattern_or_file  # pytype: disable=annotation-type-mismatch
diff --git a/openhtf/plugs/generic/serial_collection.py b/openhtf/plugs/generic/serial_collection.py
index 3b1d85f3..aa3b3a51 100644
--- a/openhtf/plugs/generic/serial_collection.py
+++ b/openhtf/plugs/generic/serial_collection.py
@@ -18,6 +18,7 @@
 
 import logging
 import threading
+from typing import Optional
 
 from openhtf.core import base_plugs
 from openhtf.util import configuration
@@ -57,10 +58,10 @@ class SerialCollectionPlug(base_plugs.BasePlug):
   # Serial library can raise these exceptions
   SERIAL_EXCEPTIONS = (serial.SerialException, ValueError)
 
-  _serial = None  # type: serial.Serial
-  _serial_port = None  # type: int
-  _collect = None  # type: bool
-  _collection_thread = None  # type: Optional[threading.Thread]
+  _serial: Optional[serial.Serial] = None
+  _serial_port: Optional[int] = None
+  _collect: Optional[bool] = None
+  _collection_thread: Optional[threading.Thread] = None
 
   @CONF.inject_positional_args
   def __init__(self, serial_collection_port, serial_collection_baud):
diff --git a/openhtf/plugs/user_input.py b/openhtf/plugs/user_input.py
index a9244e85..1d062b26 100644
--- a/openhtf/plugs/user_input.py
+++ b/openhtf/plugs/user_input.py
@@ -142,10 +142,10 @@ class UserInput(base_plugs.FrontendAwareBasePlug):
 
   def __init__(self):
     super(UserInput, self).__init__()
-    self.last_response = None  # type: Optional[Tuple[Text, Text]]
-    self._prompt = None  # type: Optional[Prompt]
-    self._console_prompt = None  # type: Optional[ConsolePrompt]
-    self._response = None  # type: Optional[Text]
+    self.last_response: Optional[tuple[str, str]] = None
+    self._prompt: Optional[Prompt] = None
+    self._console_prompt: Optional[ConsolePrompt] = None
+    self._response: Optional[Text] = None
     self._cond = threading.Condition(threading.RLock())
 
   def _asdict(self) -> Optional[Dict[Text, Any]]: