diff --git a/census/core.py b/census/core.py
index 17dc64a..2101e17 100644
--- a/census/core.py
+++ b/census/core.py
@@ -96,6 +96,11 @@ class Client(object):
     groups_url = 'https://api.census.gov/data/%s/%s/groups.json'
 
     def __init__(self, key, year=None, session=None, retries=3):
+        if key == "" or key is None:
+            raise ValueError(
+                "As of May 12, 2026, all requests to the US Census API require an API key. "
+                "You may acquire one at https://api.census.gov/data/key_signup.html"
+            )
         self._key = key
         self.session = session or new_session()
         if year:
@@ -112,7 +117,7 @@ def tables(self, year=None):
 
         # Query the table metadata as raw JSON
         tables_url = self.groups_url % (year, self.dataset)
-        resp = self.session.get(tables_url)
+        resp = self.session.get(tables_url, params={"key": self._key})
 
         # Pass it out
         return resp.json()['groups']
@@ -126,7 +131,7 @@ def fields(self, year=None, flat=False):
 
         fields_url = self.definitions_url % (year, self.dataset)
 
-        resp = self.session.get(fields_url)
+        resp = self.session.get(fields_url, params={"key": self._key})
         obj = resp.json()
 
         if flat:
@@ -216,7 +221,7 @@ def query(self, fields, geo, year=None, sort_by_geoid=False, **kwargs):
     @lru_cache(maxsize=1024)
     def _field_type(self, field, year):
         url = self.definition_url % (year, self.dataset, field)
-        resp = self.session.get(url)
+        resp = self.session.get(url, params={"key": self._key})
 
         types = {"fips-for": str,
                  "fips-in": str,
@@ -581,6 +586,11 @@ class Census(object):
     ALL = ALL
 
     def __init__(self, key, year=None, session=None):
+        if key == "" or key is None:
+            raise ValueError(
+                "As of May 12, 2026, all requests to the US Census API require an API key. "
+                "You may acquire one at https://api.census.gov/data/key_signup.html"
+            )
 
         if not session:
             session = new_session()
diff --git a/census/tests/test_census.py b/census/tests/test_census.py
index bd2df24..2286a28 100644
--- a/census/tests/test_census.py
+++ b/census/tests/test_census.py
@@ -5,7 +5,7 @@
 import unittest
 
 from census.core import (
-    Census, UnsupportedYearException)
+    Census, Client, UnsupportedYearException)
 
 KEY = os.environ.get('CENSUS_KEY', '')
 
@@ -385,5 +385,20 @@ def test_older_sf1(self):
         assert result_2010 != result_2000
 
 
+class TestAPIKeyRequired(unittest.TestCase):
+
+    def test_census_raises_without_key(self):
+        with self.assertRaises(ValueError):
+            Census(None)
+        with self.assertRaises(ValueError):
+            Census("")
+
+    def test_client_raises_without_key(self):
+        with self.assertRaises(ValueError):
+            Client(None)
+        with self.assertRaises(ValueError):
+            Client("")
+
+
 if __name__ == '__main__':
     unittest.main()