System dynamics (SD) is a powerful methodology and computer simulation modelling technique for framing, understanding and discussing complex issues and problems. It is widely used to analyse a range of systems in, e.g. business, ecology, medical and social systems as well as engineering. The methodology focuses on the way one quantity can affect others through the flow of physical entities and information. Often such flows come back to the original quantity causing a feedback loop. The behaviour of the system is governed by these feedback loops. There are two important advantages of taking systems dynamics approach. The interrelationship of the different elements of the systems can be easily seen in terms of cause and effects. Thus the true cause of the behaviour can be identified. The other advantage is that it possible to investigate which parameters or structures need to be changed in order to improve behaviour. This paper deals with the design of a framework for SD models and gives an overview of the current SD simulation packages.