A system performance model is developed to predict the performance of a high-energy laser system taking into account the propagation through the beam control system and the atmosphere. The performance model essentially shows how the various subsystems, such as the laser device, intermediate transfer system, beam expander, target, etc., will interact with each other and how a closed-loop model for the atmospheric correction can be achieved. Such a model aids in the system design by identifying and optimizing the adaptive optic requirements for the laser beam cleanup and for the atmospheric compensation correction. The heart of such a model is the Perkin-Elmer Physical Optics Propagation (POP) code. The POP code has been successfully used in many high-energy laser programs. The general capabilities of the POP code and the performance model will be highlighted. Results from a test case using a chemical laser wavefront will also be presented.