diff --git a/app/api/routers/query.py b/app/api/routers/query.py
index c5a526a..d6f0e5e 100644
--- a/app/api/routers/query.py
+++ b/app/api/routers/query.py
@@ -1,12 +1,32 @@
 """Router for query path operations."""
 
-from fastapi import APIRouter, Depends, Response, status
+from fastapi import APIRouter, Depends, HTTPException, Response, status
+from fastapi.security import OAuth2
 
-from .. import crud
+from .. import crud, security
 from ..models import CombinedQueryResponse, QueryModel
+from ..security import verify_token
+
+# from fastapi.security import open_id_connect_url
+
 
 router = APIRouter(prefix="/query", tags=["query"])
 
+# Adapted from info in https://github.com/tiangolo/fastapi/discussions/9137#discussioncomment-5157382
+oauth2_scheme = OAuth2(
+    flows={
+        "implicit": {
+            "authorizationUrl": "https://accounts.google.com/o/oauth2/auth",
+        }
+    },
+    # Don't automatically error out when request is not authenticated, to support optional authentication
+    auto_error=False,
+)
+# NOTE: Can also explicitly use OpenID Connect because Google supports it - results in the same behavior as the OAuth2 scheme above.
+# openid_connect_scheme = open_id_connect_url.OpenIdConnect(
+#     openIdConnectUrl="https://accounts.google.com/.well-known/openid-configuration"
+# )
+
 
 # We use the Response parameter below to change the status code of the response while still being able to validate the returned data using the response model.
 # (see https://fastapi.tiangolo.com/advanced/response-change-status-code/ for more info).
@@ -16,9 +36,33 @@
 # example responses for different status codes in the OpenAPI docs (less relevant for now since there is only one response model).
 @router.get("/", response_model=CombinedQueryResponse)
 async def get_query(
-    response: Response, query: QueryModel = Depends(QueryModel)
+    response: Response,
+    query: QueryModel = Depends(QueryModel),
+    token: str | None = Depends(oauth2_scheme),
 ):
     """When a GET request is sent, return list of dicts corresponding to subject-level metadata aggregated by dataset."""
+    # NOTE: Currently, when the request is unauthenticated (missing or malformed authorization header -> missing token),
+    # the default response is a 403 Forbidden error.
+    # This doesn't fully align with HTTP status code conventions:
+    # - 401 Unauthorized should be used when the client lacks authentication credentials
+    # - 403 Forbidden should be used when the client has been authenticated but lacks the required permissions
+    # If we really care about returning a 401 Unauthorized error, we can use auto_error=False
+    # when creating the OAuth2 object and raise a custom HTTPException.
+    # See also https://github.com/tiangolo/fastapi/discussions/9130
+    # if not token:
+    #     raise HTTPException(
+    #         status_code=status.HTTP_401_UNAUTHORIZED,
+    #         detail="Not authenticated",
+    #         headers={"WWW-Authenticate": "Bearer"},
+    #     )
+    if security.AUTH_ENABLED:
+        if token is None:
+            raise HTTPException(
+                status_code=status.HTTP_403_FORBIDDEN,
+                detail="Not authenticated",
+            )
+        verify_token(token)
+
     response_dict = await crud.get(
         query.min_age,
         query.max_age,
diff --git a/app/api/security.py b/app/api/security.py
new file mode 100644
index 0000000..cb159a7
--- /dev/null
+++ b/app/api/security.py
@@ -0,0 +1,43 @@
+import os
+
+from fastapi import HTTPException, status
+from fastapi.security.utils import get_authorization_scheme_param
+from google.auth.exceptions import GoogleAuthError
+from google.auth.transport import requests
+from google.oauth2 import id_token
+
+AUTH_ENABLED = os.environ.get("NB_ENABLE_AUTH", "True").lower() == "true"
+CLIENT_ID = os.environ.get("NB_QUERY_CLIENT_ID", None)
+
+
+def check_client_id():
+    """Check if the CLIENT_ID environment variable is set."""
+    # By default, if CLIENT_ID is not provided to verify_oauth2_token,
+    # Google will simply skip verifying the audience claim of ID tokens.
+    # This however can be a security risk, so we mandate that CLIENT_ID is set.
+    if AUTH_ENABLED and CLIENT_ID is None:
+        raise ValueError(
+            "Authentication has been enabled (NB_ENABLE_AUTH) but the environment variable NB_QUERY_CLIENT_ID is not set. "
+            "Please set NB_QUERY_CLIENT_ID to the Google client ID for your Neurobagel query tool deployment, to verify the audience claim of ID tokens."
+        )
+
+
+def verify_token(token: str):
+    """Verify the Google ID token. Raise an HTTPException if the token is invalid."""
+    # Adapted from https://developers.google.com/identity/gsi/web/guides/verify-google-id-token#python
+    try:
+        # Extract the token from the "Bearer" scheme
+        # (See https://github.com/tiangolo/fastapi/blob/master/fastapi/security/oauth2.py#L473-L485)
+        # TODO: Check also if scheme of token is "Bearer"?
+        _, param = get_authorization_scheme_param(token)
+        id_info = id_token.verify_oauth2_token(
+            param, requests.Request(), CLIENT_ID
+        )
+        # TODO: Remove print statement or turn into logging
+        print("Token verified: ", id_info)
+    except (GoogleAuthError, ValueError) as exc:
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail=f"Invalid token: {exc}",
+            headers={"WWW-Authenticate": "Bearer"},
+        ) from exc
diff --git a/app/main.py b/app/main.py
index e88281a..6e6de54 100644
--- a/app/main.py
+++ b/app/main.py
@@ -11,6 +11,7 @@
 
 from .api import utility as util
 from .api.routers import attributes, nodes, query
+from .api.security import check_client_id
 
 logger = logging.getLogger("nb-f-API")
 stdout_handler = logging.StreamHandler()
@@ -26,6 +27,7 @@ async def lifespan(app: FastAPI):
     """
     Collect and store locally defined and public node details for federation upon startup and clears the index upon shutdown.
     """
+    check_client_id()
     await util.create_federation_node_index()
     yield
     util.FEDERATION_NODES.clear()
diff --git a/requirements.txt b/requirements.txt
index 6314e26..ed881d1 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,13 +1,17 @@
 anyio==3.6.2
 attrs==23.1.0
+cachetools==5.3.3
 certifi==2023.7.22
 cfgv==3.3.1
+charset-normalizer==3.3.2
 click==8.1.3
+colorama==0.4.6
 coverage==7.0.0
 distlib==0.3.6
 exceptiongroup==1.0.4
 fastapi==0.95.2
 filelock==3.8.0
+google-auth==2.32.0
 h11==0.14.0
 httpcore==0.16.2
 httpx==0.23.1
@@ -22,23 +26,28 @@ orjson==3.8.6
 packaging==21.3
 pandas==1.5.2
 platformdirs==2.5.4
-pluggy==1.5.0 
+pluggy==1.0.0
 pre-commit==2.20.0
+pyasn1==0.6.0
+pyasn1_modules==0.4.0
 pydantic==1.10.2
 pyparsing==3.0.9
-pytest==8.2.1
+pytest==7.2.0
 pytest-asyncio==0.23.7
 python-dateutil==2.8.2
 pytz==2023.3.post1
 PyYAML==6.0
 referencing==0.31.1
+requests==2.31.0
 rfc3986==1.5.0
 rpds-py==0.13.2
+rsa==4.9
 six==1.16.0
 sniffio==1.3.0
 starlette==0.27.0
 toml==0.10.2
 tomli==2.0.1
 typing_extensions==4.4.0
+urllib3==2.2.0
 uvicorn==0.20.0
 virtualenv==20.16.7
diff --git a/tests/conftest.py b/tests/conftest.py
index a8b4b5f..3d67611 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -12,6 +12,33 @@ def test_app():
     yield client
 
 
+@pytest.fixture
+def disable_auth(monkeypatch):
+    """
+    Disable the authentication requirement for the API to skip startup checks
+    (for when the tested route does not require authentication).
+    """
+    monkeypatch.setattr("app.api.security.AUTH_ENABLED", False)
+
+
+@pytest.fixture()
+def mock_verify_token():
+    """Mock a successful token verification that does not raise any exceptions."""
+
+    def _verify_token(token):
+        return None
+
+    return _verify_token
+
+
+@pytest.fixture()
+def set_mock_verify_token(monkeypatch, mock_verify_token):
+    """Set the verify_token function to a mock that does not raise any exceptions."""
+    monkeypatch.setattr(
+        "app.api.routers.query.verify_token", mock_verify_token
+    )
+
+
 @pytest.fixture(scope="function")
 def set_valid_test_federation_nodes(monkeypatch):
     """Set two correctly formatted federation nodes for a test function (mocks the result of reading/parsing available public and local nodes on startup)."""
diff --git a/tests/test_nodes.py b/tests/test_nodes.py
index 3f9506f..56eb42d 100644
--- a/tests/test_nodes.py
+++ b/tests/test_nodes.py
@@ -14,7 +14,9 @@
         },
     ],
 )
-def test_nodes_discovery_endpoint(test_app, monkeypatch, local_nodes):
+def test_nodes_discovery_endpoint(
+    test_app, monkeypatch, local_nodes, disable_auth
+):
     """Test that a federation node index is correctly created from locally set and remote node lists."""
 
     def mock_parse_nodes_as_dict(path):
@@ -59,7 +61,7 @@ def mock_httpx_get(**kwargs):
 
 
 def test_failed_public_nodes_fetching_raises_warning(
-    test_app, monkeypatch, caplog
+    test_app, monkeypatch, disable_auth, caplog
 ):
     """Test that when request for remote list of public nodes fails, an informative warning is raised and the federation node index only includes local nodes."""
 
@@ -95,7 +97,7 @@ def mock_httpx_get(**kwargs):
         assert warn_substr in caplog.text
 
 
-def test_unset_local_nodes_raises_warning(test_app, monkeypatch):
+def test_unset_local_nodes_raises_warning(test_app, monkeypatch, disable_auth):
     """Test that when no local nodes are set, an informative warning is raised and the federation node index only includes remote nodes."""
 
     def mock_parse_nodes_as_dict(path):
@@ -166,7 +168,9 @@ def test_missing_local_nodes_file_does_not_raise_error(tmp_path):
     assert util.parse_nodes_as_dict(expected_file_path) == {}
 
 
-def test_no_available_nodes_raises_error(monkeypatch, test_app, caplog):
+def test_no_available_nodes_raises_error(
+    monkeypatch, test_app, disable_auth, caplog
+):
     """Test that when no local or remote nodes are available, an informative error is raised."""
 
     def mock_parse_nodes_as_dict(path):
diff --git a/tests/test_query.py b/tests/test_query.py
index b731e91..7693592 100644
--- a/tests/test_query.py
+++ b/tests/test_query.py
@@ -6,6 +6,12 @@
 from fastapi import status
 
 
+@pytest.fixture()
+def mock_token():
+    """Create a mock token that is well-formed for testing purposes."""
+    return "Bearer foo"
+
+
 @pytest.fixture()
 def mocked_single_matching_dataset_result():
     """Valid aggregate query result for a single matching dataset."""
@@ -29,6 +35,8 @@ def test_partial_node_failure_responses_handled_gracefully(
     test_app,
     set_valid_test_federation_nodes,
     mocked_single_matching_dataset_result,
+    mock_token,
+    set_mock_verify_token,
     caplog,
 ):
     """
@@ -50,7 +58,10 @@ async def mock_httpx_get(self, **kwargs):
 
     monkeypatch.setattr(httpx.AsyncClient, "get", mock_httpx_get)
 
-    response = test_app.get("/query/")
+    response = test_app.get(
+        "/query/",
+        headers={"Authorization": mock_token},
+    )
 
     assert response.status_code == status.HTTP_207_MULTI_STATUS
     assert response.json() == {
@@ -104,6 +115,8 @@ def test_partial_node_request_failures_handled_gracefully(
     test_app,
     set_valid_test_federation_nodes,
     mocked_single_matching_dataset_result,
+    mock_token,
+    set_mock_verify_token,
     error_to_raise,
     expected_node_message,
     caplog,
@@ -123,7 +136,10 @@ async def mock_httpx_get(self, **kwargs):
 
     monkeypatch.setattr(httpx.AsyncClient, "get", mock_httpx_get)
 
-    response = test_app.get("/query/")
+    response = test_app.get(
+        "/query/",
+        headers={"Authorization": mock_token},
+    )
 
     assert response.status_code == status.HTTP_207_MULTI_STATUS
 
@@ -153,6 +169,8 @@ def test_all_nodes_failure_handled_gracefully(
     monkeypatch,
     test_app,
     mock_failed_connection_httpx_get,
+    mock_token,
+    set_mock_verify_token,
     set_valid_test_federation_nodes,
     caplog,
 ):
@@ -164,7 +182,10 @@ def test_all_nodes_failure_handled_gracefully(
         httpx.AsyncClient, "get", mock_failed_connection_httpx_get
     )
 
-    response = test_app.get("/query/")
+    response = test_app.get(
+        "/query/",
+        headers={"Authorization": mock_token},
+    )
 
     # We expect 3 logs here: one warning for each failed node, and one error for the overall failure
     assert len(caplog.records) == 3
@@ -186,6 +207,8 @@ def test_all_nodes_success_handled_gracefully(
     caplog,
     set_valid_test_federation_nodes,
     mocked_single_matching_dataset_result,
+    mock_token,
+    set_mock_verify_token,
 ):
     """
     Test that when queries sent to all nodes succeed, the federation API response includes an overall success status and no errors.
@@ -201,7 +224,10 @@ async def mock_httpx_get(self, **kwargs):
 
     monkeypatch.setattr(httpx.AsyncClient, "get", mock_httpx_get)
 
-    response = test_app.get("/query/")
+    response = test_app.get(
+        "/query/",
+        headers={"Authorization": mock_token},
+    )
 
     assert response.status_code == status.HTTP_200_OK
 
@@ -210,3 +236,26 @@ async def mock_httpx_get(self, **kwargs):
     assert response["errors"] == []
     assert len(response["responses"]) == 2
     assert "Requests to all nodes succeeded (2/2)" in caplog.text
+
+
+def test_query_without_token_succeeds_when_auth_disabled(
+    monkeypatch,
+    test_app,
+    set_valid_test_federation_nodes,
+    mocked_single_matching_dataset_result,
+    disable_auth,
+):
+    """
+    Test that when authentication is disabled, a federated query request without a token succeeds.
+    """
+
+    async def mock_httpx_get(self, **kwargs):
+        return httpx.Response(
+            status_code=200, json=[mocked_single_matching_dataset_result]
+        )
+
+    monkeypatch.setattr(httpx.AsyncClient, "get", mock_httpx_get)
+
+    response = test_app.get("/query/")
+
+    assert response.status_code == status.HTTP_200_OK
diff --git a/tests/test_security.py b/tests/test_security.py
new file mode 100644
index 0000000..b657168
--- /dev/null
+++ b/tests/test_security.py
@@ -0,0 +1,64 @@
+import pytest
+from fastapi import HTTPException
+
+from app.api.security import verify_token
+
+
+def test_missing_client_id_raises_error_when_auth_enabled(
+    monkeypatch, test_app
+):
+    """Test that a missing client ID raises an error on startup when authentication is enabled."""
+    # We're using what should be default values of CLIENT_ID and AUTH_ENABLED here
+    # (if the corresponding environment variables are unset),
+    # but we set the values explicitly here for clarity
+    monkeypatch.setattr("app.api.security.CLIENT_ID", None)
+    monkeypatch.setattr("app.api.security.AUTH_ENABLED", True)
+
+    with pytest.raises(ValueError) as exc_info:
+        with test_app:
+            pass
+
+    assert "NB_QUERY_CLIENT_ID is not set" in str(exc_info.value)
+
+
+# Ignore startup warning that is unrelated to the current test
+@pytest.mark.filterwarnings(
+    "ignore:No local Neurobagel nodes defined or found"
+)
+def test_missing_client_id_ignored_when_auth_disabled(monkeypatch, test_app):
+    """Test that a missing client ID does not raise an error when authentication is disabled."""
+    monkeypatch.setattr("app.api.security.CLIENT_ID", None)
+    monkeypatch.setattr("app.api.security.AUTH_ENABLED", False)
+
+    with test_app:
+        pass
+
+
+@pytest.mark.parametrize(
+    "invalid_token",
+    ["Bearer faketoken", "Bearer", "faketoken", "fakescheme faketoken"],
+)
+def test_invalid_token_raises_error(invalid_token):
+    """Test that an invalid token raises an error from the verification process."""
+    with pytest.raises(HTTPException) as exc_info:
+        verify_token(invalid_token)
+
+    assert exc_info.value.status_code == 401
+    assert "Invalid token" in exc_info.value.detail
+
+
+@pytest.mark.parametrize(
+    "invalid_auth_header",
+    [{}, {"Authorization": ""}, {"badheader": "badvalue"}],
+)
+def test_query_with_malformed_auth_header_fails(
+    test_app, set_mock_verify_token, invalid_auth_header
+):
+    """Test that a request to the /query route with a missing or malformed authorization header, fails ."""
+
+    response = test_app.get(
+        "/query/",
+        headers=invalid_auth_header,
+    )
+
+    assert response.status_code == 403