diff --git a/include/array_view.h b/include/array_view.h
index 006f18c..26a1641 100644
--- a/include/array_view.h
+++ b/include/array_view.h
@@ -16,6 +16,9 @@
 
 #pragma once
 
+#ifndef GSL_ARRAY_VIEW_H
+#define GSL_ARRAY_VIEW_H
+
 #include <new>
 #include <stdexcept>
 #include <cstddef>
@@ -2286,3 +2289,5 @@ general_array_view_iterator<ArrayView> operator+(typename general_array_view_ite
 #endif // _MSC_VER <= 1800
 
 #pragma pop_macro("_NOEXCEPT")
+
+#endif // GSL_ARRAY_VIEW_H
diff --git a/include/fail_fast.h b/include/fail_fast.h
index dfd0ede..78a5102 100644
--- a/include/fail_fast.h
+++ b/include/fail_fast.h
@@ -16,6 +16,9 @@
 
 #pragma once
 
+#ifndef GSL_FAIL_FAST_H
+#define GSL_FAIL_FAST_H
+
 #include <exception>
 #include <stdexcept>
 
@@ -45,3 +48,5 @@ inline void fail_fast_assert(bool cond, const char* const) { if (!cond) std::ter
 #endif // SAFER_CPP_TESTING
 
 }
+
+#endif // GSL_FAIL_FAST_H
diff --git a/include/gsl.h b/include/gsl.h
index 1357d76..09a46c7 100644
--- a/include/gsl.h
+++ b/include/gsl.h
@@ -16,6 +16,9 @@
 
 #pragma once
 
+#ifndef GSL_GSL_H
+#define GSL_GSL_H
+
 #include "array_view.h"     // array_view, strided_array_view...
 #include "string_view.h"    // zstring, string_view, zstring_builder...
 #include <memory>
@@ -287,3 +290,5 @@ private:
 template<class T> using maybe_null = maybe_null_ret<T>;
 
 } // namespace Guide
+
+#endif // GSL_GSL_H
diff --git a/include/string_view.h b/include/string_view.h
index 7becc8e..e683c7a 100644
--- a/include/string_view.h
+++ b/include/string_view.h
@@ -16,6 +16,9 @@
 
 #pragma once
 
+#ifndef GSL_STRING_VIEW_H
+#define GSL_STRING_VIEW_H
+
 #include "array_view.h"
 #include <cstring>
 
@@ -176,3 +179,5 @@ using zstring_builder = basic_zstring_builder<char, Max>;
 template <size_t Max = dynamic_range>
 using wzstring_builder = basic_zstring_builder<wchar_t, Max>;
 }
+
+#endif // GSL_STRING_VIEW_H