The recent emergence of renewable energy and other cleantech industries has attracted considerable attention in innovation studies. These have emphasized the core role of processes such as entrepreneurial activities, knowledge generation and regulatory support for research and development. Processes of market formation have either been neglected or considered in aggregate terms only. We therefore propose to analyze such market formation dynamics in more detail, drawing on the technological innovation systems framework. The conceptual approach is applied to the rapid expansion of photovoltaic markets in Germany over the past twenty years.