The theme of Modelling Systems in sports includes a range of diverse papers presented at the ISEA Conference in Munich in July, 2006. Systems in sports are typically complex and include interactions between the athlete, equipment and sport which frequently result in unique outcomes. For this reason, it is necessary to develop models that represent a form of idealisation of the physical problem, but which nevertheless through manipulation and simulation provide greater insight into the particular questions of interest. The papers included in this theme involve interesting analytical, computational and experimental system models in a range of sports such as gymnastics, tennis, handball, alpine skiing, rowing, cricket, skydiving, figure skating, volleyball and football. The following section aims to provide an overview of the main research and development issues presented in these papers.