Integration is one of the pivotal concepts with respect to analyzing, designing and evaluating information systems. Integrating software components as well as integrating information systems with the surrounding action system is a core activity of most software development projects. Despite the outstanding importance of integration in Information Systems, only little work has been done on developing a conception of integration. This paper presents a conception of integration that can be applied to information systems as well as to their integration with the action systems they are supposed to support. It allows for differentiating degrees of integration. It also accounts for additional levels of integration, such as meta level or instance level integration.