diff --git a/Atomic/Mailbox.h b/Atomic/Mailbox.h
index 1251434..d4e8775 100644
--- a/Atomic/Mailbox.h
+++ b/Atomic/Mailbox.h
@@ -8,6 +8,8 @@ static_assert( __cplusplus > 2020'99 );
  * @author ADAM David Alan Martin
  */
 
+#pragma once
+
 #include <Alepha/Alepha.h>
 
 #include <vector>