This chapter presents some examples to show how simulations were used to measure, develop, and examines attributes that were of interest to an organization, but which were not measured adequately with other methods. It also presents an overview of the key concepts discussed in the subsequent chapters of this book. The book describes how to construct a wide range of different types of simulations which can be used to assess a variety of different performance dimensions. It deals with training observers because the simulation scoring process is central to simulations that call for judgmental ratings by assessors. The book provides guidance in the collection of evidence supporting the fairness, legal appropriateness, and effectiveness of simulations. It focuses on how to train various staff required for the effective implementation of simulations; and how to evaluate the effectiveness of simulations according to their intended purpose.