The widespread deployment of IT in the past decades has significantly increased the integration reach, breadth, and scope in organisations. Many of the associated socio-economic phenomena have been studied by IS researchers, for example, in the context of IT adoption, the business value of IT, and IS success. Surprisingly though, the concept of integration in itself has so far attracted only little interest on the part of researchers. According to our knowledge, no established theoretical framework seems to place integration-related constructs at the centre of scientific inquiry. The objective of the present study is to take a first step to fill this gap by reviewing the literature on integration in order to structure the existing body of knowledge and to derive an agenda for further research in this area. Our literature review reveals that, in spite of its importance for academia and practice, integration is still an under-researched topic, with a noticeable lack of theorization and synthesis of the different research strands into a more holistic model. As we argue, a to-be-developed 'Theory of Integration' would be highly valuable to increase our understanding of the different phenomena surrounding integration on the technological and the organisational level within the firm.