Integration of information systems is a prerequisite for efficient collaboration within and between organizations. Despite intensive research on integration issues in Information Systems research, companies nowadays still encounter considerable challenges in integration projects. The question for the reasons still engages researchers and practitioners. The paper at hand investigates current integration problems by deriving a framework for integration problems from literature research and applying it for the analysis of concrete cases from business practice. The examples are described in detail in explorative case studies examining three integration projects from different industries and with different objectives. From the comparison of theoretically derived and practically proven integration problems, the paper identifies integration problems that have been insufficiently addressed so far and discusses open challenges of integration in the Information Systems discipline.