Software reuse is perceived as the key to successful software development because of the potential for shortened time to market, increased quality and reduced costs. In recent years software product lines have emerged as a promising way to achieve large scale software reuse. Challenges against successful reuse when developing in a software product line involves management of the differences between products, and the differences between different releases of the products. In this thesis we present the experiences from a series of case studies within four software companies. Based on these we present a taxonomy of the technical solutions to manage product differences, a historical essay of how components in a software product line can evolve and what mechanisms that are used to support this evolution. From this we elaborate on the connection between evolution and variability, i.e. the ability of the software architecture and components to support the differences between products. We argue that evolution is strongly connected to variability, and that by foreseeing the evolution, the software can be instrumented with appropriate variability mechanisms accordingly. Moreover, we argue that some types of evolution are more frequent than others, and that the efforts should mainly go in the direction of foreseeing and instrumenting for these types of evolution.