Data is here, data is growing, data is causing Harm. Data, as distinct from software and hardware, has been a contributing factor in many accidents and incidents. The impact of data-related issues continues to grow as we rely more and more on data-driven systems. The inclusion of data as a potential cause of harm is a crucial part of a thorough safety assessment. Explicitly considering data in this way also provides additional business and societal benefit including reduced harm, reduced commercial liabilities and improved business efficiencies. The book has been developed by the SCSC Data Safety Initiative Working group (DSIWG), a cross-sector groups set up to address the challenges of data safety. It provides guidance on how data should be managed in a safety-related context. During 2018, feedback was received from four data safety courses and the application of version 3.0of the Guidance to a real aviation problem, involving the safety of Unmanned Air Vehicles. As a result of this feedback, a number of clarifications and usability enhancements have been incorporated within the document. However, all paragraph numbers remain consistent with version 3.0. This release should therefore not be seen as a new version of the Guidance, but rather as an improved version of 3.0.