Policies to address climate change by reducing greenhouse gas emissions might be made more effective if we can better understand the pathways by which transformative technologies become significant components of energy systems. Indeed, the central question of mitigation revolves around the scope of policy to influence or accelerate the diffusion of low-carbon technology. While market forces clearly influence technology deployment, understanding the longer-term and large-scale changes in the energy system requires a broader understanding of the relative influence of institutional, behavioral, and social factors. This paper presents the results of an interview-based, comparative case approach to investigating systematically the relative importance of these non-economic factors influencing technological change across technology and country contexts. We identified two low-carbon energy sectors (bioenergy and nuclear power) that underwent significant changes over the past 50 years in the energy portfolio of three countries: Brazil, Sweden, and the United States. We identified nine categories of factors that might contribute to these large technological transformations, and then evaluated, via interviews with sector participants in each country, which factors were viewed as being determinative or highly influential in the trajectory of that technology in their country context. We also draw out policy implications and directions for future research. (C) 2011 Elsevier Ltd. All rights reserved.