This article deals with system innovation in Freeman and Perez's innovation typology (incremental, radical, system, techno-economic paradigm). This article conceptualises these changes as transitions from one socio-technical system to another. These transitions are co-evolution processes that are not only about technological discontinuities, but also about markets, user practices, regulation, culture, infrastructure and science. In a critical discussion of co-evolution literatures, the article distinguishes three levels of co-evolutionary processes. To understand transitions, these insights are combined in a multi-level perspective, consisting of niche, regime and landscape levels. Transitions come about when co-evolutionary dynamics at these three levels link up and reinforce each other. The perspective is illustrated with a historical case study: the transition from aviation systems based on propeller-aircraft to aviation systems based on turbojet aircraft (1930-1970). The case study provides not just an evolutionary economic analysis of technological change, but also deals with the long-run evolution of technology and the socio-economic system. © 2005 Elsevier Ltd. All rights reserved.