This chapter indicates some of the possibilities of simulation as related to probability problems. Simulations on computers grade off into simulations in laboratories, and on in to classical laboratory experiments, which in their turn were often regarded as simulations of what would happen in the field. Most simulations are devoted to comparing alternate designs so that a good design can be found. Hence it is often the differences between simulations more than the actual numbers that matter. At present the name of Monte Carlo is used to refer to any simulation that uses random numbers. Modern computers have popularized the idea and made many simulations relatively painless. Medieval scholastic approach of deciding by mere thinking what will happen in a given situation without looking at reality. When experiment and simulation differ, that can be annoying but can also be a possibility for finding new things that the experimentalist never thought about.