diff --git a/poly2tri/common/utils.h b/poly2tri/common/utils.h
index 2fc3b5b..23c435d 100644
--- a/poly2tri/common/utils.h
+++ b/poly2tri/common/utils.h
@@ -28,6 +28,9 @@
  * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
+ 
+#ifndef UTILS_H
+#define UTILS_H
 
 #include <exception>
 #include <math.h>
@@ -95,3 +98,5 @@ bool InScanArea(Point& pa, Point& pb, Point& pc, Point& pd)
 
 }
 
+#endif
+
diff --git a/poly2tri/poly2tri.h b/poly2tri/poly2tri.h
index eab8910..487755e 100644
--- a/poly2tri/poly2tri.h
+++ b/poly2tri/poly2tri.h
@@ -28,6 +28,12 @@
  * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
+
+#ifndef POLY2TRI_H
+#define POLY2TRI_H
+
 #include "common/shapes.h"
 #include "sweep/cdt.h"
 
+#endif
+
diff --git a/poly2tri/sweep/advancing_front.h b/poly2tri/sweep/advancing_front.h
index 5b9b299..4354905 100644
--- a/poly2tri/sweep/advancing_front.h
+++ b/poly2tri/sweep/advancing_front.h
@@ -28,6 +28,10 @@
  * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
+ 
+#ifndef ADVANCED_FRONT_H
+#define ADVANCED_FRONT_H
+
 #include "../common/shapes.h"
 
 namespace p2t {
@@ -112,3 +116,4 @@ inline void AdvancingFront::set_search(Node* node)
 
 }
 
+#endif
diff --git a/poly2tri/sweep/cdt.h b/poly2tri/sweep/cdt.h
index eb007b1..9288ee6 100644
--- a/poly2tri/sweep/cdt.h
+++ b/poly2tri/sweep/cdt.h
@@ -28,6 +28,10 @@
  * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
+ 
+#ifndef CDT_H
+#define CDT_H
+
 #include "advancing_front.h"
 #include "sweep_context.h"
 #include "sweep.h"
@@ -61,3 +65,5 @@ Sweep* sweep_;
 };
 
 }
+
+#endif
diff --git a/poly2tri/sweep/sweep.h b/poly2tri/sweep/sweep.h
index bf94d61..02bf9ab 100644
--- a/poly2tri/sweep/sweep.h
+++ b/poly2tri/sweep/sweep.h
@@ -35,6 +35,9 @@
  *
  * "FlipScan" Constrained Edge Algorithm invented by Thomas Åhlén, thahlen@gmail.com
  */
+ 
+#ifndef SWEEP_H
+#define SWEEP_H
 
 namespace p2t {
 
@@ -113,3 +116,5 @@ void FinalizationPolygon(SweepContext& tcx);
 };
 
 }
+
+#endif
diff --git a/poly2tri/sweep/sweep_context.h b/poly2tri/sweep/sweep_context.h
index 6a64c0d..b70e853 100644
--- a/poly2tri/sweep/sweep_context.h
+++ b/poly2tri/sweep/sweep_context.h
@@ -28,6 +28,10 @@
  * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
+ 
+#ifndef SWEEP_CONTEXT_H
+#define SWEEP_CONTEXT_H
+
 #include <list>
 #include <vector>
 
@@ -177,3 +181,5 @@ inline Point* SweepContext::tail()
 }
 
 }
+
+#endif