In software intensive systems the integration becomes complex since both software and hardware components are integrated and run in the execution environment for the first time. Support for this stage is thus essential. Practices for Product Integration are described in different reference models. We have investigated these and compared them with activities performed in seven product development projects. Our conclusion is that descriptions of best practices in product integration are available in different reference models, but need to be merged into one set of practices. Through case studies we see that the described practices are insufficiently used in industry, and that organizations would benefit from adhering to them. Our investigations indicate that a set of practices are necessary to be successful in software product integration: define and check criteria for integration, review interface descriptions and ensure coordination of interface changes, and deliver components as agreed. In addition to these, a set of practices are supporting the integration activities, including the definition of an integration strategy, and the establishment of a suitable integration environment.