The determination of safety integrity levels (SILs) for safety instrumented functions requires the comparison of calculated risk with risk tolerance criteria. The IEC 61511/ISA 84 standard on safety instrumented systems specifies the use of risk tolerance criteria for hazardous events but does not provide any guidance on the type or form of criteria that should be used. Industry guidance on appropriate risk tolerance criteria for SIL determination is also lacking. This article discusses the type and form of criteria that should be employed and their use in SIL determination is described. (c) 2013 American Institute of Chemical Engineers Process Saf Prog 32: 307-310, 2013