Data, as distinct from software and hardware, has been a contributing factor in many accidents and incidents. The inclusion of data as a potential cause of harm is a crucial part of a thorough system safety assessment. This book has been developed by the Safety Critical Systems Club Data Safety Initiative Working Group (DSIWG) to provide guidance on how data should be managed in a safety-related context. This edition further refines and restructures the guidance to explicitly distinguish between normative, informative and discursive text to assist users who wish to claim a level of compliance with this document.