diff --git a/prune-orphans.sh b/prune-orphans.sh
new file mode 100755
index 0000000..b5562fa
--- /dev/null
+++ b/prune-orphans.sh
@@ -0,0 +1,30 @@
+#!/usr/bin/env bash
+# Remove content/<section>/*.md (zettels and bibliography) whose org source in ~/Cloud/brain no longer exists.
+# Dry run by default; pass --delete to actually remove files.
+set -euo pipefail
+
+brain=~/Cloud/brain
+content="$(cd "$(dirname "$0")" && pwd)/content"
+sections=(zettels book phdthesis inbook techreport article inproceedings websites)
+
+count=0
+for section in "${sections[@]}"; do
+  for md in "$content/$section"/*.md; do
+    [[ -e $md ]] || continue
+    name=$(basename "$md" .md)
+    [[ $name == _index ]] && continue
+    if [[ ! -f "$brain/$name.org" ]]; then
+      echo "orphan: $md"
+      [[ ${1:-} == --delete ]] && rm "$md"
+      count=$((count + 1))
+    fi
+  done
+done
+
+if [[ $count -eq 0 ]]; then
+  echo "No orphans."
+elif [[ ${1:-} != --delete ]]; then
+  echo "$count orphan(s). Re-run with --delete to remove them."
+else
+  echo "Removed $count file(s)."
+fi