diff --git a/plot.h b/plot.h
--- a/plot.h
+++ b/plot.h
@@ -17,6 +17,9 @@
   along with serialplot.  If not, see <http://www.gnu.org/licenses/>.
 */
 
+#ifndef PLOT_H
+#define PLOT_H
+
 #include <qwt_plot.h>
 #include <qwt_plot_grid.h>
 #include "zoomer.h"
@@ -46,3 +49,5 @@ public slots:
 private slots:
     void unzoomed();
 };
+
+#endif // PLOT_H
diff --git a/utils.h b/utils.h
--- a/utils.h
+++ b/utils.h
@@ -17,6 +17,9 @@
   along with serialplot.  If not, see <http://www.gnu.org/licenses/>.
 */
 
+#ifndef UTILS_H
+#define UTILS_H
+
 // credits: peppe@stackoverflow [http://stackoverflow.com/a/16795664/432492]
 template<typename... Args> struct SELECT {
     template<typename C, typename R>
@@ -24,3 +27,5 @@ template<typename... Args> struct SELECT
         return pmf;
     }
 };
+
+#endif // UTILS_H
diff --git a/version.h.in b/version.h.in
--- a/version.h.in
+++ b/version.h.in
@@ -17,8 +17,13 @@
   along with serialplot.  If not, see <http://www.gnu.org/licenses/>.
 */
 
+#ifndef VERSION_H
+#define VERSION_H
+
 #define MAJOR_VERSION    @MAJOR_VERSION@
 #define MINOR_VERSION    @MINOR_VERSION@
 #define PATCH_VERSION    @PATCH_VERSION@
 #define VERSION_STRING   "@VERSION_STRING@"
 #define VERSION_REVISION "@VERSION_REVISION@"
+
+#endif // VERSION_H