This chapter covers the emerging area of the use of commercial off-the-shelf (COTS) computer games for military, defense and security purposes. A brief background is provided of the historic link between games and military simulation, together with the size and scope of the modern computer game industry. Considerable effort is dedicated to providing a representative sample of the various defense and security usages of COTS games. Examples of current usage are drawn from a range of nations including the United States (U.S.), Australia, Denmark, Singapore and Canada. Coverage is broken into the three chief application areas of training, experimentation and decision-support, with mention of other areas such as recruitment and education. The chapter highlights the benefits and risks of the use of COTS games for defense and security purposes, including cost, acceptance, immersion, fidelity, multi-player, accessibility and rapid technological advance. The chapter concludes with a discussion of challenges and key enablers to be achieved if COTS games are to obtain their true potential as tools for defense and security training, experimentation and decision-support. Aspects highlighted include the dichotomy between games for entertainment and “serious” applications; verification, validation and accreditation; collaboration between the games industry and defense; modifiability, interoperability; quantifying training transfer; and a range of technological challenges for the games themselves.