diff --git a/src/posit/auth.py b/src/posit/auth.py
index 546281b5..44bdc00e 100644
--- a/src/posit/auth.py
+++ b/src/posit/auth.py
@@ -3,7 +3,7 @@
 
 
 class Auth(AuthBase):
-    def __init__(self, key) -> None:
+    def __init__(self, key: str) -> None:
         self.key = key
 
     def __call__(self, r: PreparedRequest) -> PreparedRequest:
diff --git a/src/posit/client.py b/src/posit/client.py
index 6e760313..a5ecae2c 100644
--- a/src/posit/client.py
+++ b/src/posit/client.py
@@ -1,22 +1,36 @@
 from requests import Session
 from typing import Optional
 
+from . import hooks
+
 from .auth import Auth
 from .config import ConfigBuilder
+from .users import Users
 
 
 class Client:
+    users: Users
+
     def __init__(
-        self, endpoint: Optional[str] = None, api_key: Optional[str] = None
+        self,
+        api_key: Optional[str] = None,
+        endpoint: Optional[str] = None,
     ) -> None:
         builder = ConfigBuilder()
-        builder.set_api_key(api_key)
-        builder.set_endpoint(endpoint)
+        if api_key:
+            builder.set_api_key(api_key)
+        if endpoint:
+            builder.set_endpoint(endpoint)
         self._config = builder.build()
+
+        if self._config.api_key is None:
+            raise ValueError("Invalid value for 'api_key': Must be a non-empty string.")
+        if self._config.endpoint is None:
+            raise ValueError(
+                "Invalid value for 'endpoint': Must be a non-empty string."
+            )
+
         self._session = Session()
+        self._session.hooks["response"].append(hooks.handle_errors)
         self._session.auth = Auth(self._config.api_key)
-
-    def get(self, endpoint: str, *args, **kwargs):  # pragma: no cover
-        return self._session.request(
-            "GET", f"{self._config.endpoint}/{endpoint}", *args, **kwargs
-        )
+        self.users = Users(self._config.endpoint, self._session)
diff --git a/src/posit/client_test.py b/src/posit/client_test.py
index 1c3ba960..d10e534a 100644
--- a/src/posit/client_test.py
+++ b/src/posit/client_test.py
@@ -1,3 +1,5 @@
+import pytest
+
 from unittest.mock import MagicMock, Mock, patch
 
 from .client import Client
@@ -24,3 +26,31 @@ def test_init(self, Auth: MagicMock, ConfigBuilder: MagicMock, Session: MagicMoc
         Session.assert_called_once()
         Auth.assert_called_once_with(api_key)
         assert client._config == config
+
+    @patch("posit.client.ConfigBuilder")
+    def test_init_without_api_key(self, ConfigBuilder: MagicMock):
+        api_key = None
+        endpoint = "http://foo.bar"
+        config = Mock()
+        config.api_key = api_key
+        config.endpoint = endpoint
+        builder = ConfigBuilder.return_value
+        builder.set_api_key = Mock()
+        builder.set_endpoint = Mock()
+        builder.build = Mock(return_value=config)
+        with pytest.raises(ValueError):
+            Client(api_key=api_key, endpoint=endpoint)
+
+    @patch("posit.client.ConfigBuilder")
+    def test_init_without_endpoint(self, ConfigBuilder: MagicMock):
+        api_key = "foobar"
+        endpoint = None
+        config = Mock()
+        config.api_key = api_key
+        config.endpoint = endpoint
+        builder = ConfigBuilder.return_value
+        builder.set_api_key = Mock()
+        builder.set_endpoint = Mock()
+        builder.build = Mock(return_value=config)
+        with pytest.raises(ValueError):
+            Client(api_key=api_key, endpoint=endpoint)
diff --git a/src/posit/config.py b/src/posit/config.py
index 534305df..ecbfc131 100644
--- a/src/posit/config.py
+++ b/src/posit/config.py
@@ -1,4 +1,5 @@
 import os
+import dataclasses
 
 from abc import ABC, abstractmethod
 from dataclasses import dataclass
@@ -20,10 +21,22 @@ def get_value(self, key: str) -> Optional[str]:
 class EnvironmentConfigProvider(ConfigProvider):
     def get_value(self, key: str) -> Optional[str]:
         if key == "api_key":
-            return os.environ.get("CONNECT_API_KEY")
+            value = os.environ.get("CONNECT_API_KEY")
+            if value:
+                return value
+            if value == "":
+                raise ValueError(
+                    "Invalid value for 'CONNECT_API_KEY': Must be a non-empty string."
+                )
 
         if key == "endpoint":
-            return os.environ.get("CONNECT_SERVER")
+            value = os.environ.get("CONNECT_SERVER")
+            if value:
+                return os.path.join(value, "__api__")
+            if value == "":
+                raise ValueError(
+                    "Invalid value for 'CONNECT_SERVER': Must be a non-empty string."
+                )
 
         return None
 
@@ -36,7 +49,8 @@ def __init__(
         self._providers = providers
 
     def build(self) -> Config:
-        for key in Config.__annotations__:
+        for field in dataclasses.fields(Config):
+            key = field.name
             if not getattr(self._config, key):
                 setattr(
                     self._config,
@@ -47,8 +61,8 @@ def build(self) -> Config:
                 )
         return self._config
 
-    def set_api_key(self, api_key: Optional[str]):
+    def set_api_key(self, api_key: str):
         self._config.api_key = api_key
 
-    def set_endpoint(self, endpoint: Optional[str]):
+    def set_endpoint(self, endpoint: str):
         self._config.endpoint = endpoint
diff --git a/src/posit/config_test.py b/src/posit/config_test.py
index 35cb58ff..10e431d2 100644
--- a/src/posit/config_test.py
+++ b/src/posit/config_test.py
@@ -1,3 +1,5 @@
+import pytest
+
 from unittest.mock import Mock, patch
 
 from .config import Config, ConfigBuilder, EnvironmentConfigProvider
@@ -10,11 +12,33 @@ def test_get_api_key(self):
         api_key = provider.get_value("api_key")
         assert api_key == "foobar"
 
+    @patch.dict("os.environ", {"CONNECT_API_KEY": ""})
+    def test_get_api_key_empty(self):
+        provider = EnvironmentConfigProvider()
+        with pytest.raises(ValueError):
+            provider.get_value("api_key")
+
+    def test_get_api_key_miss(self):
+        provider = EnvironmentConfigProvider()
+        api_key = provider.get_value("api_key")
+        assert api_key is None
+
     @patch.dict("os.environ", {"CONNECT_SERVER": "http://foo.bar"})
     def test_get_endpoint(self):
         provider = EnvironmentConfigProvider()
         endpoint = provider.get_value("endpoint")
-        assert endpoint == "http://foo.bar"
+        assert endpoint == "http://foo.bar/__api__"
+
+    @patch.dict("os.environ", {"CONNECT_SERVER": ""})
+    def test_get_endpoint_empty(self):
+        provider = EnvironmentConfigProvider()
+        with pytest.raises(ValueError):
+            provider.get_value("endpoint")
+
+    def test_get_endpoint_miss(self):
+        provider = EnvironmentConfigProvider()
+        endpoint = provider.get_value("endpoint")
+        assert endpoint is None
 
     def test_get_value_miss(self):
         provider = EnvironmentConfigProvider()
diff --git a/src/posit/errors.py b/src/posit/errors.py
new file mode 100644
index 00000000..40f777e7
--- /dev/null
+++ b/src/posit/errors.py
@@ -0,0 +1,11 @@
+class ClientError(Exception):
+    def __init__(
+        self, error_code: int, error_message: str, http_status: int, http_message: str
+    ):
+        self.error_code = error_code
+        self.error_message = error_message
+        self.http_status = http_status
+        self.http_message = http_message
+        super().__init__(
+            f"{error_message} (Error Code: {error_code}, HTTP Status: {http_status} {http_message})"
+        )
diff --git a/src/posit/errors_test.py b/src/posit/errors_test.py
new file mode 100644
index 00000000..99efcc9d
--- /dev/null
+++ b/src/posit/errors_test.py
@@ -0,0 +1,20 @@
+import pytest
+
+from .errors import ClientError
+
+
+class TestClientError:
+    def test(self):
+        error_code = 0
+        error_message = "foo"
+        http_status = 404
+        http_message = "Foo Bar"
+        with pytest.raises(
+            ClientError, match=r"foo \(Error Code: 0, HTTP Status: 404 Foo Bar\)"
+        ):
+            raise ClientError(
+                error_code=error_code,
+                error_message=error_message,
+                http_status=http_status,
+                http_message=http_message,
+            )
diff --git a/src/posit/hooks.py b/src/posit/hooks.py
new file mode 100644
index 00000000..7a41d532
--- /dev/null
+++ b/src/posit/hooks.py
@@ -0,0 +1,15 @@
+from http.client import responses
+from requests import Response
+
+from .errors import ClientError
+
+
+def handle_errors(response: Response, *args, **kwargs) -> Response:
+    if response.status_code >= 400 and response.status_code < 500:
+        data = response.json()
+        error_code = data["code"]
+        message = data["error"]
+        http_status = response.status_code
+        http_status_message = responses[http_status]
+        raise ClientError(error_code, message, http_status, http_status_message)
+    return response
diff --git a/src/posit/hooks_test.py b/src/posit/hooks_test.py
new file mode 100644
index 00000000..1ca5d754
--- /dev/null
+++ b/src/posit/hooks_test.py
@@ -0,0 +1,20 @@
+import pytest
+
+from unittest.mock import Mock
+
+from .hooks import handle_errors
+
+
+class TestHandleErrors:
+    def test(self):
+        response = Mock()
+        response.status_code = 200
+        assert handle_errors(response) == response
+
+    def test_client_error(self):
+        response = Mock()
+        response.status_code = 400
+        response.json = Mock()
+        response.json.return_value = {"code": 0, "error": "foobar"}
+        with pytest.raises(Exception):
+            handle_errors(response)
diff --git a/src/posit/users.py b/src/posit/users.py
new file mode 100644
index 00000000..ade6a8bb
--- /dev/null
+++ b/src/posit/users.py
@@ -0,0 +1,17 @@
+import os
+
+from requests import Session, Response
+
+
+class Users:
+    def __init__(self, endpoint: str, session: Session) -> None:
+        self._endpoint = endpoint
+        self._session = session
+
+    def get_user(self, user_id: str) -> Response:
+        endpoint = os.path.join(self._endpoint, "v1/users", user_id)
+        return self._session.get(endpoint)
+
+    def get_current_user(self) -> Response:
+        endpoint = os.path.join(self._endpoint, "v1/user")
+        return self._session.get(endpoint)
diff --git a/src/posit/users_test.py b/src/posit/users_test.py
new file mode 100644
index 00000000..b79463e8
--- /dev/null
+++ b/src/posit/users_test.py
@@ -0,0 +1,21 @@
+from unittest.mock import Mock
+
+from .users import Users
+
+
+class TestUsers:
+    def test_get_user(self):
+        session = Mock()
+        session.get = Mock(return_value={})
+        users = Users(endpoint="http://foo.bar/", session=session)
+        response = users.get_user(user_id="foo")
+        assert response == {}
+        session.get.assert_called_once_with("http://foo.bar/v1/users/foo")
+
+    def test_get_current_user(self):
+        session = Mock()
+        session.get = Mock(return_value={})
+        users = Users(endpoint="http://foo.bar/", session=session)
+        response = users.get_current_user()
+        assert response == {}
+        session.get.assert_called_once_with("http://foo.bar/v1/user")
diff --git a/tinkering.py b/tinkering.py
new file mode 100644
index 00000000..d6fb000d
--- /dev/null
+++ b/tinkering.py
@@ -0,0 +1,5 @@
+from posit.client import Client
+
+client = Client()
+res = client.users.get_current_user()
+print(res.json())