During the last two decades there has been a great deal of research on renewable energy technologies. It is commonly thought that very little has come out of this research in terms of commercially interesting technologies. The first objective of this paper is to demonstrate that this perception is no longer entirely correct; in the 1990s there has been a double-digit growth rate in the market for some renewable energy technologies. The consequent alteration in the energy system, is, however, a slow, painful and highly uncertain process. This process, we argue, needs to be studied using an innovation system perspective where the focus is on networks, institutions and firms' perceptions, competencies and strategies. The second objective of the paper is therefore to present the bare bones of such an analytical framework. A third objective is to identify a set of key issues related to the speed and direction of that transformation process which needs to be studied further.