diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 3774d64..2b5d2c7 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -28,6 +28,8 @@ jobs:
         run: pylint pro_wes/ setup.py
       - name: Check code format with Black
         run: black --check setup.py pro_wes/ tests/
+      - name: Check types with mypy
+        run: mypy setup.py pro_wes/
   test:
     name: Run tests
     runs-on: ubuntu-latest
diff --git a/mypy.ini b/mypy.ini
new file mode 100644
index 0000000..2ee57fc
--- /dev/null
+++ b/mypy.ini
@@ -0,0 +1,17 @@
+[mypy]
+
+[mypy-foca.*]
+ignore_missing_imports = True
+disable_error_code = attr-defined
+
+[mypy-celery.*]
+ignore_missing_imports = True
+
+[mypy-connexion.*]
+ignore_missing_imports = True
+
+[mypy-pymongo.*]
+ignore_missing_imports = True
+
+[mypy-bson.objectid]
+ignore_missing_imports = True
\ No newline at end of file
diff --git a/pro_wes/app.py b/pro_wes/app.py
index 3d9a07f..66aa05a 100644
--- a/pro_wes/app.py
+++ b/pro_wes/app.py
@@ -29,15 +29,15 @@ def _setup_first_start(app: App) -> None:
     """Set up application for first start."""
     with app.app.app_context():
         # create storage directory
-        work_dir = Path(current_app.config.foca.custom.post_runs.storage_path.resolve())
+        _config_foca = current_app.config.foca  # type: ignore
+        work_dir = Path(_config_foca.custom.post_runs.storage_path.resolve())
         work_dir.mkdir(parents=True, exist_ok=True)
         # set service info
-        service_info = ServiceInfo()
         try:
-            service_info = service_info.get_service_info()
+            ServiceInfo().get_service_info()
         except NotFound:
-            service_info.set_service_info(
-                data=current_app.config.foca.custom.service_info.dict()
+            ServiceInfo().set_service_info(
+                data=current_app.config.foca.custom.service_info.dict()  # type: ignore
             )
 
 
diff --git a/pro_wes/exceptions.py b/pro_wes/exceptions.py
index b7965c7..b4ac405 100644
--- a/pro_wes/exceptions.py
+++ b/pro_wes/exceptions.py
@@ -39,6 +39,10 @@ class StorageUnavailableProblem(OSError):
     """Storage unavailable for OS operations."""
 
 
+class WesEndpointProblem(NotFound):
+    """No/few reuirements provided for WES endpoint."""
+
+
 exceptions = {
     Exception: {
         "message": "An unexpected error occurred.",
@@ -96,4 +100,8 @@ class StorageUnavailableProblem(OSError):
         "message": "Storage is not accessible.",
         "code": "500",
     },
+    WesEndpointProblem: {
+        "message": "No/few reuirements provided for WES endpoint.",
+        "code": "500",
+    },
 }
diff --git a/pro_wes/ga4gh/wes/models.py b/pro_wes/ga4gh/wes/models.py
index 8737785..0e0a32f 100644
--- a/pro_wes/ga4gh/wes/models.py
+++ b/pro_wes/ga4gh/wes/models.py
@@ -38,10 +38,12 @@ class Attachment(BaseModel):
 
     Args:
         filename: Name of the file as indicated in the run request.
+        object: File object.
         path: Path to the file on the app's storage system.
     """
 
     filename: str
+    object: bytes
     path: Path
 
 
@@ -156,7 +158,7 @@ def json_serialized_object_field_valid(  # pylint: disable=no-self-argument
                 value does not represent an object/dictionary.
         """
         if value == "" or value == "null" or value is None:
-            if field.name == "workflow_params":
+            if field == "workflow_params":
                 raise ValueError("field required")
             return "{}"
         try:
@@ -171,7 +173,7 @@ def json_serialized_object_field_valid(  # pylint: disable=no-self-argument
     def workflow_type_and_version_supported(  # pylint: disable=no-self-argument
         cls,
         values: Dict,
-    ) -> str:
+    ) -> Dict:
         """Ensure that workflow type and version are supported by this service
         instance.
 
@@ -338,7 +340,7 @@ class WesEndpoint(BaseModel):
 
     host: str
     base_path: Optional[str] = "/ga4gh/wes/v1"
-    run_id: Optional[str]
+    run_id: Optional[str] = None
 
 
 class DbDocument(BaseModel):
diff --git a/pro_wes/ga4gh/wes/service_info.py b/pro_wes/ga4gh/wes/service_info.py
index f333811..3b30db2 100644
--- a/pro_wes/ga4gh/wes/service_info.py
+++ b/pro_wes/ga4gh/wes/service_info.py
@@ -30,7 +30,7 @@ class ServiceInfo:
     def __init__(self) -> None:
         """Class constructor."""
         self.config: Dict = current_app.config
-        self.foca_config: Config = self.config.foca
+        self.foca_config: Config = self.config.foca  # type: ignore
         self.db_client_service_info: Collection = (
             self.foca_config.db.dbs["runStore"].collections["service_info"].client
         )
diff --git a/pro_wes/ga4gh/wes/workflow_runs.py b/pro_wes/ga4gh/wes/workflow_runs.py
index 60bd5bf..d3fd27e 100644
--- a/pro_wes/ga4gh/wes/workflow_runs.py
+++ b/pro_wes/ga4gh/wes/workflow_runs.py
@@ -26,6 +26,7 @@
     RunNotFound,
     StorageUnavailableProblem,
     Unauthorized,
+    WesEndpointProblem,
 )
 from pro_wes.ga4gh.wes.models import (
     Attachment,
@@ -55,7 +56,7 @@ class WorkflowRuns:
     def __init__(self) -> None:
         """Class constructor."""
         self.config: Dict = current_app.config
-        self.foca_config: Config = current_app.config.foca
+        self.foca_config: Config = current_app.config.foca  # type: ignore
         self.db_client: Collection = (
             self.foca_config.db.dbs["runStore"].collections["runs"].client
         )
@@ -91,11 +92,20 @@ def run_workflow(
         document.user_id = kwargs.get("user_id", None)
 
         # create run environment & insert run document into run collection
-        document_stored = self._create_run_environment(document=document)
+        document_stored: DbDocument = self._create_run_environment(document=document)
 
         # write workflow attachments
         self._save_attachments(attachments=document_stored.attachments)
 
+        # ensure WES endpoint is available
+        assert document_stored.wes_endpoint is not None, "No WES endpoint available."
+        assert (
+            document_stored.wes_endpoint.base_path is not None
+        ), "WES endpoint does not have base_path."
+
+        if document_stored.task_id is None:
+            raise IdsUnavailableProblem
+
         # instantiate WES client
         wes_client: WesClient = WesClient(
             host=document_stored.wes_endpoint.host,
@@ -136,25 +146,36 @@ def run_workflow(
             if response.status_code == 403:
                 raise Forbidden
             raise InternalServerError
-        document_stored: DbDocument = db_connector.upsert_fields_in_root_object(
+        updated_document_stored: DbDocument = db_connector.upsert_fields_in_root_object(
             root="wes_endpoint",
             run_id=response.run_id,
         )
 
+        # ensure WES endpoint is available
+        assert (
+            updated_document_stored.wes_endpoint is not None
+        ), "No WES endpoint available."
+        assert (
+            updated_document_stored.wes_endpoint.base_path is not None
+        ), "WES endpoint does not have base_path."
+
+        if updated_document_stored.wes_endpoint.run_id is None:
+            raise WesEndpointProblem
+
         # track workflow progress in background
         task__track_run_progress.apply_async(
             None,
             {
                 "jwt": kwargs.get("jwt", None),
-                "remote_host": document_stored.wes_endpoint.host,
-                "remote_base_path": document_stored.wes_endpoint.base_path,
-                "remote_run_id": document_stored.wes_endpoint.run_id,
+                "remote_host": updated_document_stored.wes_endpoint.host,
+                "remote_base_path": updated_document_stored.wes_endpoint.base_path,
+                "remote_run_id": updated_document_stored.wes_endpoint.run_id,
             },
-            task_id=document_stored.task_id,
+            task_id=updated_document_stored.task_id,
             soft_time_limit=controller_config.timeout_job,
         )
 
-        return {"run_id": document_stored.run_log.run_id}
+        return {"run_id": updated_document_stored.wes_endpoint.run_id}
 
     def list_runs(
         self,
@@ -393,7 +414,7 @@ def _create_run_environment(
                 charset=controller_config.id_charset,
                 length=controller_config.id_length,
             )
-            work_dir = Path(controller_config.storage_path).resolve() / run_id
+            work_dir: Path = Path(controller_config.storage_path).resolve() / run_id
 
             # try to create working directory
             try:
@@ -406,7 +427,7 @@ def _create_run_environment(
             # populate document
             document.run_log.run_id = run_id
             document.task_id = uuid()
-            document.work_dir = str(work_dir)
+            document.work_dir = work_dir
             document.attachments = self._process_attachments(
                 work_dir=work_dir,
             )
@@ -437,18 +458,17 @@ def _process_attachments(self, work_dir: Path) -> List[Attachment]:
         attachments = []
         files = request.files.getlist("workflow_attachment")
         for file in files:
+            assert file is not None, "File object cannot be None."
+            assert file.filename is not None, "File does not have a filename."
+
             attachments.append(
                 Attachment(
                     filename=file.filename,
-                    object=file.stream,
-                    path=str(
-                        work_dir
-                        / self._secure_filename(
-                            name=Path(file.filename),
-                        )
-                    ),
+                    object=file.stream.read(),
+                    path=work_dir / self._secure_filename(name=Path(file.filename)),
                 )
             )
+
         return attachments
 
     @staticmethod
diff --git a/pro_wes/tasks/track_run_progress.py b/pro_wes/tasks/track_run_progress.py
index 625750e..2cf9f7b 100644
--- a/pro_wes/tasks/track_run_progress.py
+++ b/pro_wes/tasks/track_run_progress.py
@@ -18,6 +18,7 @@
 from pro_wes.utils.db import DbDocumentConnector
 from pro_wes.ga4gh.wes.client_wes import WesClient
 from pro_wes.celery_worker import celery
+from pro_wes.exceptions import WesEndpointProblem
 
 logger = logging.getLogger(__name__)
 
@@ -28,7 +29,7 @@
     ignore_result=True,
     track_started=True,
 )
-def task__track_run_progress(
+def task__track_run_progress(  # pylint: disable=too-many-statements
     self,
     remote_host: str,
     remote_base_path: str,
@@ -56,7 +57,7 @@ def task__track_run_progress(
         pro_wes.exceptions.EngineUnavailable: The remote service is unavailable
             or is not a valid WES service.
     """
-    foca_config: Config = current_app.config.foca
+    foca_config: Config = current_app.config.foca  # type: ignore
     controller_config: Dict = foca_config.custom.post_runs
 
     logger.info(f"[{self.request.id}] Start processing...")
@@ -87,38 +88,43 @@ def task__track_run_progress(
     try:
         # workaround for cwl-WES; add .dict() when cwl-WES response conforms
         # to model
-        response = wes_client.get_run(run_id=remote_run_id)
+        response: RunLog = wes_client.get_run(run_id=remote_run_id)
     except EngineUnavailable:
         db_client.update_run_state(state=State.SYSTEM_ERROR.value)
         raise
     #    if not isinstance(response, RunLog):
     #        db_client.update_run_state(state=State.SYSTEM_ERROR.value)
     #        raise EngineProblem("Did not receive expected response.")
-    response.pop("request", None)
+    response.pop("request", None)  # type: ignore
     document: DbDocument = db_client.upsert_fields_in_root_object(
         root="run_log",
-        **response,
+        **response.dict(),
     )
 
     # track workflow run progress
     run_state: State = State.UNKNOWN
     attempt: int = 1
     while not run_state.is_finished:
-        sleep(controller_config.polling_wait)
+        sleep(controller_config.polling_wait)  # type: ignore
+
+        # ensure WES endpoint is available
+        assert document.wes_endpoint is not None, "No WES endpoint available."
+        if document.wes_endpoint.run_id is None:
+            raise WesEndpointProblem
         try:
-            response = wes_client.get_run_status(
+            wes_client.get_run_status(
                 run_id=document.wes_endpoint.run_id,
                 timeout=foca_config.custom.defaults.timeout,
             )
         except EngineUnavailable as exc:
-            if attempt <= controller_config.polling_attempts:
+            if attempt <= controller_config.polling_attempts:  # type: ignore
                 attempt += 1
                 logger.warning(exc, exc_info=True)
                 continue
             db_client.update_run_state(state=State.SYSTEM_ERROR.value)
             raise
         if not isinstance(response, RunStatus):
-            if attempt <= controller_config.polling_attempts:
+            if attempt <= controller_config.polling_attempts:  # type: ignore
                 attempt += 1
                 logger.warning(f"Received error response: {response}")
                 continue
@@ -129,6 +135,8 @@ def task__track_run_progress(
             run_state = response.state
             db_client.update_run_state(state=run_state.value)
 
+    assert response.run_id is not None, "WES run ID not available."
+
     # fetch run log and upsert database document
     try:
         # workaround for cwl-WES; add .dict() when cwl-WES response conforms
@@ -140,10 +148,11 @@ def task__track_run_progress(
     #    if not isinstance(response, RunLog):
     #        db_client.update_run_state(state=State.SYSTEM_ERROR.value)
     #        raise EngineProblem("Did not receive expected response.")
-    response.pop("request", None)
+    response.pop("request", None)  # type: ignore
     document = db_client.upsert_fields_in_root_object(
         root="run_log",
-        **response,
+        **dict(response),
     )
 
     logger.info(f"[{self.request.id}] Processing completed.")
+    return self.request.id
diff --git a/pro_wes/utils/db.py b/pro_wes/utils/db.py
index bbadcf5..8482909 100644
--- a/pro_wes/utils/db.py
+++ b/pro_wes/utils/db.py
@@ -88,7 +88,7 @@ def upsert_fields_in_root_object(
         self,
         root: str,
         **kwargs,
-    ) -> Optional[Mapping]:
+    ) -> DbDocument:
         """Insert (or update) fields in(to) the same root object and return
         document.
         """
diff --git a/requirements_dev.txt b/requirements_dev.txt
index 965b86d..fb589bd 100644
--- a/requirements_dev.txt
+++ b/requirements_dev.txt
@@ -5,3 +5,7 @@ mongomock>=4.1.2,<5
 pylint>=2.15.5,<3
 pytest>=7.2.0,<8
 python-semantic-release>=7.32.2,<8
+mypy~=1.8
+types-setuptools~=69.0.0.20240106
+types-requests~=2.31.0.20240106
+types-docutils~=0.20.0.20240106
\ No newline at end of file
diff --git a/setup.py b/setup.py
index 071492e..33a2896 100644
--- a/setup.py
+++ b/setup.py
@@ -16,7 +16,7 @@
 
 setup(
     name="pro-wes",
-    version=__version__,  # noqa: F821  # pylint: disable=undefined-variable
+    version=__version__,  # type: ignore # noqa: E501 F821 # pylint: disable=undefined-variable
     license="Apache License 2.0",
     description="Proxy/gateway GA4GH WES service",
     long_description=LONG_DESCRIPTION,
diff --git a/tests/ga4gh/wes/endpoints/test_service_info.py b/tests/ga4gh/wes/endpoints/test_service_info.py
index db68dc9..7d94ba5 100644
--- a/tests/ga4gh/wes/endpoints/test_service_info.py
+++ b/tests/ga4gh/wes/endpoints/test_service_info.py
@@ -11,7 +11,7 @@
 
 from unittest.mock import MagicMock
 
-from pro_wes.ga4gh.wes.service_info import RegisterServiceInfo
+from pro_wes.ga4gh.wes.service_info import RegisterServiceInfo  # type: ignore
 from pro_wes.exceptions import (
     NotFound,
     ValidationError,