This article addresses the topic of radical innovation, making two additions to the strategic niche management (SNM) approach, which conceptualises how innovations cross the 'valley of death'. First, it articulates the theoretical foundations of SNM, theorizing the connections between social constructivist and evolutionary theories of technical change. Second, the resulting socio-cognitive evolution perspective is used to explain the different patterns in biogas development in the Netherlands and Denmark. In both countries, biogas experienced ups and downs, but the timing and shape of developments differ. A longitudinal, comparative case analysis shows that the socio-cognitive evolution perspective is useful in explaining these differences. © 2009 Elsevier Ltd. All rights reserved.