The Monte Carlo simulation method has found widespread application in the radiological sciences. Its popularization as an investigational tool is demonstrated by the number of papers in Medical Physics and Physics in Medicine and Biology which utilize Monte Carlo simulation techniques (approximately a fivefold increase since 1983). This expansion is perhaps most likely due to the computing power which is easily accessible to the radiological physicist. At present, it is no longer necessary to consider performing Monte Carlo simulation solely on large mainframe computer systems. In addition, the presence and availability of software packages (e.g., ETRAN, OGRE-G, MORSE) has increased the utilization of the Monte Carlo simulation technique. In this chapter, we briefly review the chronology of the development of Monte Carlo simulation and describe the fundamentals of the technique.