This paper addresses the integration of immigrants and other societal groups from a conceptual point of view. There are many operationalizations of the concept of integration in the literature, but the fundamental question of how integration should be conceptualized is rarely addressed. This makes it difficult to assess whether a particular operationalization is suitable for measuring integration. The aim of this paper is to develop a formal understanding of the concept of integration, and not to review the existing literature. With increased conceptual clarity and by working toward an objective definition as far as possible, the paper creates a possible foundation on which indicators of integration can be built. Integration is conceptualized as proximity, and a distinction is drawn between the integration of groups and individuals. It is argued that integration should be understood as assimilation in relevant dimensions, whereas in other dimension significant differences are accepted.