diff --git a/cas-template.md b/cas-template.md
index ca15775..4d81ca0 100644
--- a/cas-template.md
+++ b/cas-template.md
@@ -12,6 +12,7 @@ For a given blob digest, consumers MUST provide at least the following variables
 * `encoded`, matching `encoded` in the `digest` rule.
 
 and expand the URI Template as defined in [RFC 6570 section 3][rfc6570-s3].
+If the expanded URI reference is a relative reference, it MUST be resolved following [RFC 3986 section 5][rfc3986-s5].
 
 ## Example
 
@@ -35,5 +36,6 @@ so the expanded URI is:
     https://a.example.com/cas/sha256/e3/e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
 
 [digest]: https://github.com/opencontainers/image-spec/blob/v1.0.0/descriptor.md#digests
+[rfc3986-s5]: https://tools.ietf.org/html/rfc3986#section-5
 [rfc6570]: https://tools.ietf.org/html/rfc6570
 [rfc6570-s3]: https://tools.ietf.org/html/rfc6570#section-3
diff --git a/index-template.md b/index-template.md
index c91cf0b..02b29ec 100644
--- a/index-template.md
+++ b/index-template.md
@@ -14,6 +14,7 @@ Consumers MUST provide at least the following variables:
     If `fragment` was not provided in the image name, it defaults to an empty string.
 
 and expand the URI Template as defined in [RFC 6570 section 3][rfc6570-s3].
+If the expanded URI reference is a relative reference, it MUST be resolved following [RFC 3986 section 5][rfc3986-s5].
 
 The server providing the expanded URI MUST support requests for media type [`application/vnd.oci.image.index.v1+json`][index].
 Servers MAY support other media types using HTTP content negotiation, as described in [RFC 7231 section 3.4][rfc7231-s3.4] (which is [also supported over HTTP/2][rfc7540-s8]).
@@ -88,6 +89,7 @@ Deciding whether to look for `1.0` (the `fragment`) or the full `a.b.example.com
 [index]: https://github.com/opencontainers/image-spec/blob/v1.0.0/image-index.md
 [index.json]: https://github.com/opencontainers/image-spec/blob/v1.0.0/image-layout.md#indexjson-file
 [rfc2606-s3]: https://tools.ietf.org/html/rfc2606#section-3
+[rfc3986-s5]: https://tools.ietf.org/html/rfc3986#section-5
 [rfc6570]: https://tools.ietf.org/html/rfc6570
 [rfc6570-s3]: https://tools.ietf.org/html/rfc6570#section-3
 [rfc7231-s3.4]: https://tools.ietf.org/html/rfc7231#section-3.4
diff --git a/oci_discovery/ref_engine/oci_index_template.py b/oci_discovery/ref_engine/oci_index_template.py
index c582f61..bb0699b 100644
--- a/oci_discovery/ref_engine/oci_index_template.py
+++ b/oci_discovery/ref_engine/oci_index_template.py
@@ -14,6 +14,7 @@
 
 import logging as _logging
 import pprint as _pprint
+import urllib.parse as _urllib_parse
 
 try:
     import uritemplate as _uritemplate
@@ -34,12 +35,15 @@ def __str__(self):
             self.__class__.__name__,
             self.uri_template)
 
-    def __init__(self, uri):
+    def __init__(self, uri, base=None):
         self.uri_template = _uritemplate.URITemplate(uri=uri)
+        self.base = base
 
     def resolve(self, name):
         name_parts = _host_based_image_names.parse(name=name)
         uri = self.uri_template.expand(**name_parts)
+        if self.base:
+            uri = _urllib_parse.urljoin(base=self.base, url=uri)
         _LOGGER.debug('fetching an OCI index for {} from {}'.format(name, uri))
         index = _fetch_json.fetch(
             uri=uri,
diff --git a/oci_discovery/ref_engine/test_oci_index_template.py b/oci_discovery/ref_engine/test_oci_index_template.py
index f893269..c5f6e66 100644
--- a/oci_discovery/ref_engine/test_oci_index_template.py
+++ b/oci_discovery/ref_engine/test_oci_index_template.py
@@ -135,3 +135,67 @@ def test_bad(self):
                         return_value=response):
                     generator = engine.resolve(name='example.com/a')
                     self.assertRaisesRegex(error, regex, list, generator)
+
+    def test_reference_expansion(self):
+        response = {
+            'manifests': [
+                {
+                    'entry': 'a',
+                    'annotations': {
+                        'org.opencontainers.image.ref.name': '1.0',
+                    },
+                },
+            ],
+        }
+        for uri, base, expected in [
+                    (
+                        'index.json',
+                        'https://example.com/a',
+                        'https://example.com/index.json',
+                    ),
+                    (
+                        'index.json',
+                        'https://example.com/a/',
+                        'https://example.com/a/index.json',
+                    ),
+                    (
+                        'https://{host}/{path}#{fragment}',
+                        'https://a.example.com/b/',
+                        'https://example.com/a#1.0'
+                    ),
+                    (
+                        '//{host}/{path}#{fragment}',
+                        'https://a.example.com/b/',
+                        'https://example.com/a#1.0'
+                    ),
+                    (
+                        '/{path}#{fragment}',
+                        'https://b.example.com/c/',
+                        'https://b.example.com/a#1.0',
+                    ),
+                    (
+                        '{path}#{fragment}',
+                        'https://b.example.com/c/',
+                        'https://b.example.com/c/a#1.0',
+                    ),
+                    (
+                        '#{fragment}',
+                        'https://example.com/a',
+                        'https://example.com/a#1.0',
+                    ),
+                    (
+                        '#{fragment}',
+                        'https://example.com/a/',
+                        'https://example.com/a/#1.0',
+                    ),
+                ]:
+            with self.subTest(label='{} from {}'.format(uri, base)):
+                engine = oci_index_template.Engine(uri=uri, base=base)
+                with unittest.mock.patch(
+                        target='oci_discovery.ref_engine.oci_index_template._fetch_json.fetch',
+                        return_value=response) as mock:
+                    resolved = list(engine.resolve(name='example.com/a#1.0'))
+                    mock.assert_called_with(
+                        uri=expected,
+                        media_type='application/vnd.oci.image.index.v1+json')
+                self.assertEqual(resolved, response['manifests'])