Studies on complex systems have shown the effect of feed backs, delays, and reinforcements, i. e. control processes. An analysismethod considering this fact has to make use of a model consisting of interacting feedback loops, as they actually determine the system behaviour. System Dynamics Models are Simulation models of coupled nonlinear differential equations which are solved approximatively on a digital computer as coupled difference equations. The method of System Dynamics offers the possibility of analyzing the structure and the behaviour of complex systems with economic, ecologic, social, and technical elements under particular consideration of its feedback structure. Prior to the introduction of this method, basic definitions are given. For illustration, a paradigma of a simple model is shown in the appendix by demonstration of the dynamic behaviour of spreading diseases