JWST will have 5 to 10 times the collecting area of the Hubble Space Telescope, making it the largest optical system put into space by a large margin. Moreover, to keep the self-emission of the optics and detectors below the background, JWST will operate at cryogenic temperatures. In addition, JWST will operate at L2, where servicing is not a viable option; therefore, we must know that the JWST performs the way we expect it to before it launches. This requires establishing a high-confidence integration and verification process for a very large, cryogenic system while remaining affordable. This paper describes the verification process planned for JWST and the trades that led to this plan. The verification considers the all-important optical performance of the telescope itself, as well as the sunshield, the wavefront sensing and control system, the vibration isolation system, the spacecraft, and the end-to-end (photons in to data storage on the ground) performance verification. We discuss testing configurations, metrology methods, facilities, and the role of analytical modeling and testing at various stages of integration to ensure the performance is understood.