Risk matrices are used widely in process safety to rate and rank risks posed by processes to help with decision making. For example, commonly they are used in process hazard analysis to rate the risks of hazard scenarios and determine the need for risk reduction. However, there are no standards for risk matrices in process safety. Companies develop their own without the benefit of established industry guidelines. Risk matrices are deceptively simple but their design and use are rife with pitfalls, even for experienced users. Some of these pitfalls are not obvious and invalid risk ratings can result which are unrecognized. Guidelines for constructing risk matrices that address these pitfalls are provided. (c) 2017 American Institute of Chemical Engineers Process Process Saf Prog 37:49-55, 2018