Abstract Mathematical models and simulation are used in several areas of systems engineering. In research and development they are used in studies of process internal phenomena like flow, mixing, reactions, heat and mass transfer, etc. to gain a thorough understanding of what is really going on inside the process. In product design models are used in defining effects of variables on product quality and amount. In process design, modelling, simulation and optimization methods are nowadays used in studying alternatives for process equipment and connections, to optimize process operation and to find best ways to utilize raw materials and energy. Equipment sizing also utilizes models and simulation. In control engineering, models and simulation are utilized in determining control strategies for the process. Simulation is also an interesting tool for process operation. It is employed in disturbance and alarm analysis, in start-up and shut-down planning, in real time control and optimization and in operator training. This material was written for the Process Simulation Course given to two graduate schools during winter 1996: Graduate School in Chemical Engineering (GSCE) and International Ph.D. Program in Pulp and Paper Science and Technology.