diff --git a/effects.h b/effects.h
index f4abb86..0c3be8a 100644
--- a/effects.h
+++ b/effects.h
@@ -14,6 +14,9 @@
  *  along with this program.  If not, see <http://www.gnu.org/licenses>.
  */
 
+#ifndef GDIGI_EFFECTS_H
+#define GDIGI_EFFECTS_H
+
 #include <glib/gtypes.h>
 
 typedef struct {
@@ -45,3 +48,5 @@ typedef struct {
     Effect *effect; /* list of supported effects */
     gint amt;       /* list of supported effects length */
 } EffectList;
+
+#endif /* GDIGI_EFFECTS_H */
diff --git a/gdigi.h b/gdigi.h
index 394d139..2105332 100644
--- a/gdigi.h
+++ b/gdigi.h
@@ -14,6 +14,9 @@
  *  along with this program.  If not, see <http://www.gnu.org/licenses>.
  */
 
+#ifndef GDIGI_H
+#define GDIGI_H
+
 #include <glib.h>
 
 enum {
@@ -462,3 +465,5 @@ void set_option(guint id, guint position, guint value);
 void switch_user_preset(int x);
 void switch_system_preset(int x);
 void set_preset_level(int level);
+
+#endif /* GDIGI_H */
diff --git a/gui.h b/gui.h
index 62710ce..c925d28 100644
--- a/gui.h
+++ b/gui.h
@@ -14,4 +14,9 @@
  *  along with this program.  If not, see <http://www.gnu.org/licenses>.
  */
 
+#ifndef GDIGI_GUI_H
+#define GDIGI_GUI_H
+
 void create_window();
+
+#endif /* GDIGI_GUI_H */
diff --git a/tests.h b/tests.h
index d65a97f..69821ae 100644
--- a/tests.h
+++ b/tests.h
@@ -14,6 +14,9 @@
  *  along with this program.  If not, see <http://www.gnu.org/licenses>.
  */
 
+#ifndef GDIGI_TESTS_H
+#define GDIGI_TESTS_H
+
 #include "gdigi.h"
 
 void test_wah();
@@ -29,3 +32,5 @@ void test_reverb();
 void test_exp();
 void test_usb();
 void test_all();
+
+#endif /* GDIGI_TESTS_H */