Utility systems have many connections to processes, both physical and electrical. This interconnectivity introduces common mode failures and the potential for crosscontamination. This article uses "real-plant" incidents to illustrate what can happen when the hazards of interactions between process and utility systems are not identified or understood. Some tools to evaluate interactions between process and utility systems are discussed. The reactivity matrix tool, which can help evaluate crosscontamination issues, is introduced. Individual types of utility systems and issues specific to them are discussed. (c) 2007 American Institute of Chemical Engineers Process.