In the field of human factors, simulation can serve a variety of important purposes. For example, driving simulators can help assess a driver’s fitness to drive under dangerous roadway conditions, and flight simulators can help train pilots to perform complicated flight maneuvers. Critically, these goals can be accomplished without putting the driver or pilot at risk for a crash that might harm themselves or others. Simulation can also be a valuable design tool. This chapter explores how simulation can aid the design process, introduces relevant concepts and issues to consider when using simulation for design, and presents two case studies in which simulation methods were used by CREATE investigators to understand, and also solve, design challenges involving older adults.