The purpose of this technical manual is to provide a basic introduction to and overview of the subject of reliability. It is particularly written for personnel involved with the acquisition and support of Command, Control, Communication, Computer, Intelligence, Surveillance, and Reconnaissance (C4ISR) equipment. The information in this manual reflects the theoretical and practical aspects of the reliability discipline. It includes information from commercial practices and lessons learned over many years of developing and implementing reliability programs for a wide variety of systems and equipment. Although some theory is presented, it is purposely limited and kept as simple as possible.