diff --git a/blag/blag.py b/blag/blag.py
index c770489..4633e56 100644
--- a/blag/blag.py
+++ b/blag/blag.py
@@ -218,6 +218,7 @@ def build(args: argparse.Namespace) -> None:
     """
     os.makedirs(f"{args.output_dir}", exist_ok=True)
     convertibles = []
+    known_targets = []
     for root, dirnames, filenames in os.walk(args.input_dir):
         for filename in filenames:
             rel_src = os.path.relpath(
@@ -229,15 +230,22 @@ def build(args: argparse.Namespace) -> None:
                 rel_dst = rel_src
                 rel_dst = rel_dst[:-3] + ".html"
                 convertibles.append((rel_src, rel_dst))
+                known_targets.append(
+                    os.path.abspath(f"{args.output_dir}/{rel_dst}")
+                )
             else:
                 shutil.copy(
                     f"{args.input_dir}/{rel_src}",
                     f"{args.output_dir}/{rel_src}",
                 )
+                known_targets.append(
+                    os.path.abspath(f"{args.output_dir}/{rel_src}")
+                )
         for dirname in dirnames:
             # all directories are copied into the output directory
             path = os.path.relpath(f"{root}/{dirname}", start=args.input_dir)
             os.makedirs(f"{args.output_dir}/{path}", exist_ok=True)
+            known_targets.append(os.path.abspath(f"{args.output_dir}/{path}"))
 
     # copy static files over
     logger.info("Copying static files.")
@@ -273,6 +281,24 @@ def build(args: argparse.Namespace) -> None:
         article_template,
     )
 
+    # clean up files that should not be there
+    for root, dirnames, filenames in os.walk(args.output_dir):
+        for filename in filenames:
+            dst = os.path.abspath(f"{root}/{filename}")
+            if dst not in known_targets:
+                logger.info(f"deleting {dst}")
+                os.remove(dst)
+            else:
+                known_targets.remove(dst)
+        for dirname in dirnames:
+            dst = os.path.abspath(f"{root}/{dirname}")
+            if dst not in known_targets:
+                logger.info(f"deleting {dst}")
+                shutil.rmtree(dst)
+            else:
+                known_targets.remove(dst)
+    logger.debug(known_targets)
+
     generate_feed(
         articles,
         args.output_dir,
diff --git a/tests/test_blag.py b/tests/test_blag.py
index 155e483..cd4bf71 100644
--- a/tests/test_blag.py
+++ b/tests/test_blag.py
@@ -312,6 +312,22 @@ def test_build(args: Namespace) -> None:
     assert os.path.exists(f"{args.output_dir}/tags/bar.html")
 
 
+def test_remove_extra_files(args):
+    """Test that extra files are removed."""
+    # create a file and directory in output dir that have no corresponding
+    # source
+    file_path = f'{args.output_dir}/a'
+    dir_path = f'{args.output_dir}/b'
+    fh = open(file_path, 'w')
+    fh.close()
+    os.mkdir(dir_path)
+
+    blag.build(args)
+
+    assert not os.path.exists(file_path)
+    assert not os.path.exists(dir_path)
+
+
 @pytest.mark.parametrize(
     "template",
     [