A safety instrumented system (SIS) may fail to operate as desired when one or more of its devices fail due to random, systematic, and common cause events. IEC 61511 (ANSI/ISA 84.00.01-2004) stresses the importance of minimizing the propagation of device failure into system failure through design, operating, inspection, and maintenance practices. To fully understand the lifecycle requirements, it is first necessary to understand the types of failures and their potential effects on the SIS. Although several technical standards and other specialized literature address the topic, it is still a "fuzzy" matter, subject to misunderstanding and discussion. IEC 61511 Clause 11.9 requires that the SIL be verified using quantitative analysis, such as reliability block diagrams, fault tree analysis, and Markov modeling. This analysis includes only those dangerous failures that are random in nature. Common cause failures may or may not be included in the verification calculation depending on whether they exhibit random or systematic behavior. Any personnel assigned responsibility for verifying the SIL should understand each failure type and the strategies that can be used against it. Consequently, this article provides an overview of random, systematic, and common cause failures and clarifies the differences in their management within IEC 61511.(C) 2006 American Institute of Chemical Engineers.