Simulation plays an increasingly important role in the development of new missile systems. This paper contains a brief overview of the various types of simulation models used in different phases of design and evaluation. The main emphasis is placed on trajectory simulation models. The usefulness of different trajectory models for different purposes is treated. A recommendation is to avoid using more complicated models than are required to address the problem of interest. The problem of using a very limited number of test firings to validate a highly complex model is mentioned.