An attempt is made to sort out the wide variety of concepts used in connection with systems integration and to give an overview of the field. Four aspects of systems integration are addressed: integration technology, integration architecture, semantic integration, and user integration. It is concluded that systems integration is difficult and complicated, and that there are no obvious shortcuts. In addition, integration is made difficult by most existing systems being not particularly integration friendly. The authors give some guidelines on how systems should be built to be more integration friendly. They also give some recommendations on how existing systems can be made more integration friendly. >