diff --git a/api/any.hpp b/api/any.hpp
index 01482c60c..9f5525cfb 100644
--- a/api/any.hpp
+++ b/api/any.hpp
@@ -7,7 +7,7 @@
 
 #include <string>
 #include <typeinfo>
-#include <stddef.h>
+#include <cstddef>
 #include <cstdlib>
 #include <cstring>
 #include <ostream>
diff --git a/api/fn.hpp b/api/fn.hpp
index f56c313f1..bae49a296 100644
--- a/api/fn.hpp
+++ b/api/fn.hpp
@@ -6,7 +6,7 @@
 #define __JULE_FN_HPP
 
 #include <string>
-#include <stddef.h>
+#include <cstddef>
 #include <functional>
 #include <thread>
 
diff --git a/api/slice.hpp b/api/slice.hpp
index 649cecf52..50a918095 100644
--- a/api/slice.hpp
+++ b/api/slice.hpp
@@ -5,7 +5,7 @@
 #ifndef __JULE_SLICE_HPP
 #define __JULE_SLICE_HPP
 
-#include <stddef.h>
+#include <cstddef>
 #include <sstream>
 #include <ostream>
 #include <initializer_list>
diff --git a/api/types.hpp b/api/types.hpp
index e56b321a8..cf1ba062e 100644
--- a/api/types.hpp
+++ b/api/types.hpp
@@ -5,7 +5,7 @@
 #ifndef __JULE_TYPES_HPP
 #define __JULE_TYPES_HPP
 
-#include <stddef.h>
+#include <cstddef>
 #include <ostream>
 
 #include "platform.hpp"
diff --git a/api/utf16.hpp b/api/utf16.hpp
index 9258183d6..9f0100865 100644
--- a/api/utf16.hpp
+++ b/api/utf16.hpp
@@ -14,7 +14,7 @@
 // Based on std::unicode::utf16
 //
 
-#include <stddef.h>
+#include <cstddef>
 #include <tuple>
 #include <string>