diff --git a/include/ndtbl/axis.hpp b/include/ndtbl/axis.hpp
index ec7ecef..3f4e335 100644
--- a/include/ndtbl/axis.hpp
+++ b/include/ndtbl/axis.hpp
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: MIT
+
 #pragma once
 
 #include "ndtbl/types.hpp"
diff --git a/include/ndtbl/detail/binary_io.hpp b/include/ndtbl/detail/binary_io.hpp
index c7cda48..81f6e31 100644
--- a/include/ndtbl/detail/binary_io.hpp
+++ b/include/ndtbl/detail/binary_io.hpp
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: MIT
+
 #pragma once
 
 #include "ndtbl/detail/size_math.hpp"
diff --git a/include/ndtbl/detail/mapped_payload.hpp b/include/ndtbl/detail/mapped_payload.hpp
index 82d5efb..2ce0857 100644
--- a/include/ndtbl/detail/mapped_payload.hpp
+++ b/include/ndtbl/detail/mapped_payload.hpp
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: MIT
+
 #pragma once
 
 #ifndef NDTBL_ENABLE_MMAP
diff --git a/include/ndtbl/detail/size_math.hpp b/include/ndtbl/detail/size_math.hpp
index 88a1cfd..25ab566 100644
--- a/include/ndtbl/detail/size_math.hpp
+++ b/include/ndtbl/detail/size_math.hpp
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: MIT
+
 #pragma once
 
 #include <cstddef>
diff --git a/include/ndtbl/diagnostics.hpp b/include/ndtbl/diagnostics.hpp
index 7f5be09..ccf9966 100644
--- a/include/ndtbl/diagnostics.hpp
+++ b/include/ndtbl/diagnostics.hpp
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: MIT
+
 #pragma once
 
 #include <cstddef>
diff --git a/include/ndtbl/field_group.hpp b/include/ndtbl/field_group.hpp
index 1883144..da84c5a 100644
--- a/include/ndtbl/field_group.hpp
+++ b/include/ndtbl/field_group.hpp
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: MIT
+
 #pragma once
 
 #include "ndtbl/detail/mapped_payload.hpp"
diff --git a/include/ndtbl/grid.hpp b/include/ndtbl/grid.hpp
index c952dcc..8622d6a 100644
--- a/include/ndtbl/grid.hpp
+++ b/include/ndtbl/grid.hpp
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: MIT
+
 #pragma once
 
 #include "ndtbl/axis.hpp"
diff --git a/include/ndtbl/io.hpp b/include/ndtbl/io.hpp
index 67d7620..9b7f2d8 100644
--- a/include/ndtbl/io.hpp
+++ b/include/ndtbl/io.hpp
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: MIT
+
 #pragma once
 
 #include "ndtbl/detail/binary_io.hpp"
diff --git a/include/ndtbl/metadata.hpp b/include/ndtbl/metadata.hpp
index b410bbd..55fc628 100644
--- a/include/ndtbl/metadata.hpp
+++ b/include/ndtbl/metadata.hpp
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: MIT
+
 #pragma once
 
 #include "ndtbl/axis.hpp"
diff --git a/include/ndtbl/ndtbl.hpp b/include/ndtbl/ndtbl.hpp
index e2ef7c7..4beeb48 100644
--- a/include/ndtbl/ndtbl.hpp
+++ b/include/ndtbl/ndtbl.hpp
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: MIT
+
 #pragma once
 
 #include "ndtbl/axis.hpp"
diff --git a/include/ndtbl/payload.hpp b/include/ndtbl/payload.hpp
index 8dc5268..21a29d7 100644
--- a/include/ndtbl/payload.hpp
+++ b/include/ndtbl/payload.hpp
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: MIT
+
 #pragma once
 
 #include <cstddef>
diff --git a/include/ndtbl/runtime_field_group.hpp b/include/ndtbl/runtime_field_group.hpp
index 1301bab..ac2a8b5 100644
--- a/include/ndtbl/runtime_field_group.hpp
+++ b/include/ndtbl/runtime_field_group.hpp
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: MIT
+
 #pragma once
 
 #include "ndtbl/field_group.hpp"
diff --git a/include/ndtbl/types.hpp b/include/ndtbl/types.hpp
index a58bd24..16d9a5e 100644
--- a/include/ndtbl/types.hpp
+++ b/include/ndtbl/types.hpp
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: MIT
+
 #pragma once
 
 #include <cstdint>