The NLN Jeffries Simulation Theory, Second Edition establishes the substantive foundation essential for research, education, and practice, and advances knowledge in the area of simulation. Created through a systematic process involving robust research and literature review, along with nursing perspectives from those immersed in simulations, this work can be an effective tool or guide to implementation as well as further research. This Second Edition provides an updated review of the theory, which explains the phenomenon of simulation, presents an updated systematic review of the theoretical constructs and uses of the theory, and highlights future research needs in the area of clinical simulation. This mid-range theory provides a way to study the phenomenon of simulation that can facilitate the exploration of best practices, outcomes, and system change through research and development that will contribute further to the discovery of new knowledge and practices.