diff --git a/bin/cuirass.in b/bin/cuirass.in
index 43f4661..9d8a39e 100644
--- a/bin/cuirass.in
+++ b/bin/cuirass.in
@@ -28,7 +28,8 @@ exec ${GUILE:-@GUILE@} --no-auto-compile -e main -s "$0" "$@"
              (guix derivations)
              (guix store)
              (ice-9 getopt-long)
-             (ice-9 popen))
+             (ice-9 popen)
+             (ice-9 rdelim))
 
 (define (show-help)
   (format #t "Usage: ~a [OPTIONS] SPECFILE~%" (%program-name))
@@ -54,6 +55,13 @@ exec ${GUILE:-@GUILE@} --no-auto-compile -e main -s "$0" "$@"
 (define (fetch-repository spec)
   "Get the latest version of repository specified in SPEC.  Clone repository
 if required."
+  (define (current-commit)
+    (let* ((pipe   (open-input-pipe "git log -n1"))
+           (log    (read-string pipe))
+           (commit (cadr (string-split log char-set:whitespace))))
+      (close-pipe pipe)
+      commit))
+
   (let ((cachedir (%package-cachedir)))
     (or (file-exists? cachedir) (mkdir cachedir))
     (with-directory-excursion cachedir
@@ -68,7 +76,8 @@ if required."
                (zero? (system* "git" "reset" "--hard"
                                (or tag
                                    commit
-                                   (string-append "origin/" branch))))))))))
+                                   (string-append "origin/" branch))))
+               (current-commit)))))))
 
 (define (compile dir)
   ;; Required for fetching Guix bootstrap tarballs.
@@ -116,13 +125,16 @@ if required."
 (define (process-specs db jobspecs)
   "Evaluate and build JOBSPECS and store results in DB."
   (for-each (λ (spec)
-              (fetch-repository spec)
-              (compile (string-append (%package-cachedir) "/"
-                                      (assq-ref spec #:name)))
-              (with-store store
-                (let ((jobs (evaluate store db spec)))
-                  (set-build-options store #:use-substitutes? #f)
-                  (build-packages store db jobs))))
+              (let ((commit (fetch-repository spec))
+                    (stamp  (db-get-stamp db spec)))
+                (unless (string=? commit stamp)
+                  (compile (string-append (%package-cachedir) "/"
+                                          (assq-ref spec #:name)))
+                  (with-store store
+                    (let ((jobs (evaluate store db spec)))
+                      (set-build-options store #:use-substitutes? #f)
+                      (build-packages store db jobs))))
+                (db-add-stamp db spec commit)))
             jobspecs))
 
 
diff --git a/src/cuirass/database.scm b/src/cuirass/database.scm
index 3b8ffb9..dbbe00a 100644
--- a/src/cuirass/database.scm
+++ b/src/cuirass/database.scm
@@ -29,6 +29,8 @@
             db-close
             db-add-specification
             db-get-specifications
+            db-add-stamp
+            db-get-stamp
             evaluation-exists?
             db-add-evaluation
             db-get-evaluation
@@ -188,3 +190,23 @@ INSERT INTO Builds (derivation, log, output) VALUES ('~A', '~A', '~A');"
                (assq-ref build #:log)
                (assq-ref build #:output))
   (last-insert-rowid db))
+
+(define (db-get-stamp db spec)
+  "Return a stamp corresponding to specification SPEC in database DB."
+  (let ((res (sqlite-exec db "SELECT * FROM Stamps WHERE specification='~A';"
+                          (assq-ref spec #:id))))
+    (match res
+      (() "")
+      ((#(spec commit)) commit))))
+
+(define (db-add-stamp db spec commit)
+  "Associate stamp COMMIT to specification SPEC in database DB."
+  (if (string-null? (db-get-stamp db spec))
+      (sqlite-exec db "\
+INSERT INTO Stamps (specification, stamp) VALUES ('~A', '~A');"
+                   (assq-ref spec #:id)
+                   commit)
+      (sqlite-exec db "\
+UPDATE Stamps SET stamp='~A' WHERE specification='~A';"
+                   commit
+                   (assq-ref spec #:id))))
diff --git a/src/schema.sql b/src/schema.sql
index 9cc7167..d5c1f00 100644
--- a/src/schema.sql
+++ b/src/schema.sql
@@ -14,6 +14,12 @@ CREATE TABLE Specifications (
   revision      TEXT
 );
 
+CREATE TABLE Stamps (
+  specification INTEGER NOT NULL PRIMARY KEY,
+  stamp         TEXT NOT NULL,
+  FOREIGN KEY (specification) REFERENCES Specifications (id)
+);
+
 CREATE TABLE Evaluations (
   derivation    TEXT NOT NULL PRIMARY KEY,
   job_name      TEXT NOT NULL,