Energy transitions to sustainability receive much interest in politics and science. Using a socio-technical and multi-level theory on transitions, this article draws important lessons from a long-term analysis of the Dutch electricity system. The article analyses technical developments, changes in rules and visions, and social networks that support and oppose renewable options. The article is multi-level because it looks at novel renewable energy technologies and structural trends in the existing electricity regime. The analysis shows that an energy transition, with roots in the 1960s and 1970s, is already occurring, but driven mainly by liberalisation and Europeanisation. Environmental aspects have become part of this ongoing transition, but do not form its main driver. Many barriers exist for a sustainability transition, but there are also some opportunities. A long-term analysis of renewable niche-innovation trajectories (wind, biomass, PV) provides lessons about socio-technical dynamics, problems and windows of opportunity. © 2006 Elsevier Ltd. All rights reserved.