diff --git a/README.md b/README.md
index 50f2481..e10a595 100644
--- a/README.md
+++ b/README.md
@@ -24,6 +24,10 @@ client = Client(
 )
 ```
 
+`access_token` accepts both personal access tokens (`ldpat_…`) and service
+account tokens (`ldsvc_…`); the SDK selects the correct authentication scheme
+automatically.
+
 ## Development
 
 ### Prerequisites
diff --git a/lightdash/client.py b/lightdash/client.py
index 2e44768..09d65a2 100644
--- a/lightdash/client.py
+++ b/lightdash/client.py
@@ -33,6 +33,7 @@ def __init__(self, instance_url: str, access_token: str, project_uuid: str, conf
         self.instance_url = instance_url.rstrip('/')
         self.access_token = access_token
         self.project_uuid = project_uuid
+        self.auth_header = "Bearer" if access_token.startswith("ldsvc_") else "ApiKey"
         
         # Extract config values with defaults
         config = config or {}
@@ -88,7 +89,7 @@ def _make_request(
         
         with httpx.Client(
             headers={
-                "Authorization": f"ApiKey {self.access_token}",
+                "Authorization": f"{self.auth_header} {self.access_token}",
                 "Accept": "application/json",
             },
             timeout=self.timeout
diff --git a/tests/test_client.py b/tests/test_client.py
new file mode 100644
index 0000000..f749849
--- /dev/null
+++ b/tests/test_client.py
@@ -0,0 +1,38 @@
+"""
+Unit tests for Client construction.
+
+Covers the Authorization scheme selection: service account tokens (``ldsvc_``)
+authenticate with ``Bearer`` while personal access tokens and anything else use
+``ApiKey`` — matching the Lightdash backend auth middleware (issue #17).
+"""
+
+import pytest
+from lightdash import Client
+
+
+def _client(token: str) -> Client:
+    # Construction makes no network call, so a dummy URL/project is fine.
+    return Client(
+        instance_url="https://example.lightdash.cloud",
+        access_token=token,
+        project_uuid="00000000-0000-0000-0000-000000000000",
+    )
+
+
+class TestAuthHeaderScheme:
+    def test_service_account_token_uses_bearer(self):
+        """ldsvc_ tokens must use the Bearer scheme."""
+        assert _client("ldsvc_abc123").auth_header == "Bearer"
+
+    def test_personal_access_token_uses_apikey(self):
+        """ldpat_ tokens keep the ApiKey scheme (unchanged behaviour)."""
+        assert _client("ldpat_abc123").auth_header == "ApiKey"
+
+    def test_unprefixed_token_defaults_to_apikey(self):
+        """Any other token defaults to ApiKey, preserving backwards compatibility."""
+        assert _client("legacy-token").auth_header == "ApiKey"
+
+    def test_authorization_header_value(self):
+        """The composed Authorization header uses the selected scheme."""
+        c = _client("ldsvc_abc123")
+        assert f"{c.auth_header} {c.access_token}" == "Bearer ldsvc_abc123"