The purpose of this Guide is to assist development and maintenance projects (teams and individuals) that have to address the use of, or consideration of, COTS products within large, complex systems, including but not limited to mission critical systems. This assistance is provided by capturing a set of information about COTS products (benefits, risks, recommended practices, lifecycle activity impacts) and mission critical systems (variety of MCS, special needs for MCS, differences between MCS and other types of systems) and then providing some linkage between these topics so that various types of stakeholders can find useful information. The document should be of value to both management and technical individuals/teams. It should also be of value to teams that are dealing with non-MCS, in that the scope is not limited to only MCS.