diff --git a/kiss_fft.h b/kiss_fft.h
index bcf3b5d..45c3a6a 100644
--- a/kiss_fft.h
+++ b/kiss_fft.h
@@ -1,3 +1,11 @@
+/*
+ *  Copyright (c) 2003-2010, Mark Borgerding. All rights reserved.
+ *  This file is part of KISS FFT - https://github.com/mborgerding/kissfft
+ *
+ *  SPDX-License-Identifier: BSD-3-Clause
+ *  See COPYING file for more information.
+ */
+
 #ifndef KISS_FFT_H
 #define KISS_FFT_H
 
diff --git a/kissfft.hh b/kissfft.hh
index 4f6ac92..43ab00b 100644
--- a/kissfft.hh
+++ b/kissfft.hh
@@ -1,3 +1,11 @@
+/*
+ *  Copyright (c) 2003-2010, Mark Borgerding. All rights reserved.
+ *  This file is part of KISS FFT - https://github.com/mborgerding/kissfft
+ *
+ *  SPDX-License-Identifier: BSD-3-Clause
+ *  See COPYING file for more information.
+ */
+
 #ifndef KISSFFT_CLASS_HH
 #define KISSFFT_CLASS_HH
 #include <complex>
diff --git a/tools/kfc.h b/tools/kfc.h
index 9b5fd67..a5cdd16 100644
--- a/tools/kfc.h
+++ b/tools/kfc.h
@@ -1,3 +1,11 @@
+/*
+ *  Copyright (c) 2003-2004, Mark Borgerding. All rights reserved.
+ *  This file is part of KISS FFT - https://github.com/mborgerding/kissfft
+ *
+ *  SPDX-License-Identifier: BSD-3-Clause
+ *  See COPYING file for more information.
+ */
+
 #ifndef KFC_H
 #define KFC_H
 #include "kiss_fft.h"
diff --git a/tools/kiss_fftnd.h b/tools/kiss_fftnd.h
index 42e7df5..9bbc002 100644
--- a/tools/kiss_fftnd.h
+++ b/tools/kiss_fftnd.h
@@ -1,3 +1,11 @@
+/*
+ *  Copyright (c) 2003-2004, Mark Borgerding. All rights reserved.
+ *  This file is part of KISS FFT - https://github.com/mborgerding/kissfft
+ *
+ *  SPDX-License-Identifier: BSD-3-Clause
+ *  See COPYING file for more information.
+ */
+
 #ifndef KISS_FFTND_H
 #define KISS_FFTND_H
 
diff --git a/tools/kiss_fftndr.h b/tools/kiss_fftndr.h
index 38ec3ab..472e479 100644
--- a/tools/kiss_fftndr.h
+++ b/tools/kiss_fftndr.h
@@ -1,3 +1,11 @@
+/*
+ *  Copyright (c) 2003-2004, Mark Borgerding. All rights reserved.
+ *  This file is part of KISS FFT - https://github.com/mborgerding/kissfft
+ *
+ *  SPDX-License-Identifier: BSD-3-Clause
+ *  See COPYING file for more information.
+ */
+
 #ifndef KISS_NDR_H
 #define KISS_NDR_H
 
diff --git a/tools/kiss_fftr.h b/tools/kiss_fftr.h
index 48c9084..588948d 100644
--- a/tools/kiss_fftr.h
+++ b/tools/kiss_fftr.h
@@ -1,3 +1,11 @@
+/*
+ *  Copyright (c) 2003-2004, Mark Borgerding. All rights reserved.
+ *  This file is part of KISS FFT - https://github.com/mborgerding/kissfft
+ *
+ *  SPDX-License-Identifier: BSD-3-Clause
+ *  See COPYING file for more information.
+ */
+
 #ifndef KISS_FTR_H
 #define KISS_FTR_H