diff --git a/include/mbedtls/debug.h b/include/mbedtls/debug.h
index d859dd5..7d944f3 100644
--- a/include/mbedtls/debug.h
+++ b/include/mbedtls/debug.h
@@ -1,7 +1,7 @@
 /**
  * \file debug.h
  *
- * \brief Debug functions
+ * \brief Functions for controlling and providing debug output from the library.
  *
  *  Copyright (C) 2006-2015, ARM Limited, All Rights Reserved
  *  SPDX-License-Identifier: Apache-2.0
@@ -80,39 +80,141 @@
 #endif
 
 /**
- * \brief   Set the level threshold to handle globally. Messages that have a
- *          level over the threshold value are ignored.
- *          (Default value: 0 (No debug))
+ * \brief   Set the threshold error level to handle globally all debug output.
+ *          Debug messages that have a level over the threshold value are
+ *          discarded.
+ *          (Default value: 0 = No debug )
  *
- * \param threshold     maximum level of messages to pass on
+ * \param threshold     theshold level of messages to filter on. Messages at a
+ *                      higher level will be discarded.
+ *                          - Debug levels
+ *                              - 0 No debug
+ *                              - 1 Error
+ *                              - 2 State change
+ *                              - 3 Informational
+ *                              - 4 Verbose
  */
 void mbedtls_debug_set_threshold( int threshold );
 
+/**
+* \brief    Print a message to the debug output. This function is always used
+ *          through the MBEDTLS_SSL_DEBUG_MSG() macro, which supplies the ssl
+ *          context, file and line number parameters.
+ *
+ * \param ssl       SSL context
+ * \param level     error level of the debug message
+ * \param file      file the message has occurred in
+ * \param line      line number the message has occurred at
+ * \param format    format specifier, in printf format
+ * \param ...       variables used by the format specifier
+ *
+ * \attention       This function is intended for INTERNAL usage within the
+ *                  library only.
+ */
 void mbedtls_debug_print_msg( const mbedtls_ssl_context *ssl, int level,
                               const char *file, int line,
                               const char *format, ... );
 
+/**
+ * \brief   Print the return value of a function to the debug output. This
+ *          function is always used through the MBEDTLS_SSL_DEBUG_RET() macro,
+ *          which supplies the ssl context, file and line number parameters.
+ *
+ * \param ssl       SSL context
+ * \param level     error level of the debug message
+ * \param file      file the error has occurred in
+ * \param line      line number the error has occurred in
+ * \param text      the name of the function that returned the error
+ * \param ret       the return code value
+ *
+ * \attention       This function is intended for INTERNAL usage within the
+ *                  library only.
+ */
 void mbedtls_debug_print_ret( const mbedtls_ssl_context *ssl, int level,
                       const char *file, int line,
                       const char *text, int ret );
 
+/**
+ * \brief   Output a buffer of size len bytes to the debug output. This function
+ *          is always used through the MBEDTLS_SSL_DEBUG_BUF() macro,
+ *          which supplies the ssl context, file and line number parameters.
+ *
+ * \param ssl       SSL context
+ * \param level     error level of the debug message
+ * \param file      file the error has occurred in
+ * \param line      line number the error has occurred in
+ * \param text      a name or label for the buffer being dumped. Normally the
+ *                  variable or buffer name
+ * \param buf       the buffer to be outputted
+ * \param len       length of the buffer
+ *
+ * \attention       This function is intended for INTERNAL usage within the
+ *                  library only.
+ */
 void mbedtls_debug_print_buf( const mbedtls_ssl_context *ssl, int level,
                       const char *file, int line, const char *text,
                       const unsigned char *buf, size_t len );
 
 #if defined(MBEDTLS_BIGNUM_C)
+/**
+ * \brief   Print a MPI variable to the debug output. This function is always
+ *          used through the MBEDTLS_SSL_DEBUG_MPI() macro, which supplies the
+ *          ssl context, file and line number parameters.
+ *
+ * \param ssl       SSL context
+ * \param level     error level of the debug message
+ * \param file      file the error has occurred in
+ * \param line      line number the error has occurred in
+ * \param text      a name or label for the MPI being output. Normally the
+ *                  variable name
+ * \param X         the MPI variable
+ *
+ * \attention       This function is intended for INTERNAL usage within the
+ *                  library only.
+ */
 void mbedtls_debug_print_mpi( const mbedtls_ssl_context *ssl, int level,
                       const char *file, int line,
                       const char *text, const mbedtls_mpi *X );
 #endif
 
 #if defined(MBEDTLS_ECP_C)
+/**
+ * \brief   Print an ECP point to the debug output. This function is always
+ *          used through the MBEDTLS_SSL_DEBUG_ECP() macro, which supplies the
+ *          ssl context, file and line number parameters.
+ *
+ * \param ssl       SSL context
+ * \param level     error level of the debug message
+ * \param file      file the error has occurred in
+ * \param line      line number the error has occurred in
+ * \param text      a name or label for the ECP point being output. Normally the
+ *                  variable name
+ * \param X         the ECP point
+ *
+ * \attention       This function is intended for INTERNAL usage within the
+ *                  library only.
+ */
 void mbedtls_debug_print_ecp( const mbedtls_ssl_context *ssl, int level,
                       const char *file, int line,
                       const char *text, const mbedtls_ecp_point *X );
 #endif
 
 #if defined(MBEDTLS_X509_CRT_PARSE_C)
+/**
+ * \brief   Print a X.509 certificate structure to the debug output. This
+ *          function is always used through the MBEDTLS_SSL_DEBUG_CRT() macro,
+ *          which supplies the ssl context, file and line number parameters.
+ *
+ * \param ssl       SSL context
+ * \param level     error level of the debug message
+ * \param file      file the error has occurred in
+ * \param line      line number the error has occurred in
+ * \param text      a name or label for the certificate being output
+ * \param crt       X.509 certificate structure
+ *
+ * \attention       This function is intended for INTERNAL usage within the
+ *                  library only.
+ */
 void mbedtls_debug_print_crt( const mbedtls_ssl_context *ssl, int level,
                       const char *file, int line,
                       const char *text, const mbedtls_x509_crt *crt );
@@ -123,3 +225,4 @@
 #endif
 
 #endif /* debug.h */
+