Modern business applications consist of many subsystems (or components) potentially developed and maintained by diverse organizations. Generally, there are three different points of view. First, organizations using business applications are interested in the unified look and feel of composed applications, the maximum interoperability and synergetic features among subsystems, the high availability of all subsystems, and quick and seamless updates after new releases or bug fixes. Second, organizations providing single subsystems want, on the one hand, of course, to satisfy their customers and business partners, but on the other hand, to minimize their overall effort. Third, organizations integrating single subsystems aim at a uniform and cost-efficient integration architecture. This chapter takes the two latter viewpoints and describes a methodology for organizations integrating their subsystems with many business applications and all relevant types of subsystems, as well as with the whole family of subsystems from different vendors. The methodology is a product-line approach optimally tailored to the needs of such organizations. It views subsystems delivered by a single organization with all corresponding integration contexts and requirements as a family of similar systems, and engineers this family by taking systematical advantage of common characteristics and proactively considering differences in anticipated future scenarios. The methodology is based on Fraunhofer PuLSE™ (PuLSE™ is a trademark of the Fraunhofer Gesellschaft), a customizable product-line approach validated in practice by many industry organizations since 1997. The integration methodology has been developed in the German research project UNIVERSYS by tailoring Fraunhofer PuLSE™ together with industry partners to the integration context described.