It is increasingly clear that data, not just systems and software, can present a safety problem; consequently the UK Safety Critical Systems Club has promoted efforts to look at the issue of data in safety systems. This started with a seminar ‘How to stop data causing harm’ held in December 2012 and has progressed with regular meetings of a working group during 2013 and into 2014. The group, comprising industry, academic, government and independent consultants has produced this guidance document. The objective of the guidance is to describe the data safety problem and initially, for key data types and lifecycles, provide methods for defining the level of risk and recommend strategies and methods for safety risk reduction.