This collection of articles by an international panel of experts deals with the question of simulation techniques in nuclear reactor technology. In nuclear power plant, considerations of safety are always dominant. Simulation techniques are used in the analysis and design of safe plant, making this probably the most important and most actively developing topic in nuclear power-plant technology today. Modern engineering practice relies on the use of conceptual models as a convenient and efficient way of using our accumulated and often complex stock of knowledge. Simulation is one such area of conceptual model building, and is generally taken to refer to the representation of time-varying processes often in real time. Simulation techniques invariably involve the use of computers, and, among other things, can be used to model details of processes for design and analytical purposes and in the modelling of the real time behaviour of a full-scale plant.