The author looks at diverse concepts and roles of trust in the challenge of decarbonising energy systems, drawing on 25 years of personal experience in the fields of energy and environmental policy research. The paper focuses on three issues-public trust in science, institutional trust in making technology choices, and the idea that high-trust societies are more sustainable than those exhibiting low-trust. While trust is a key concept in understanding the public acceptability of technology choices, it is only one of a suite of interrelated concepts that must be addressed, which also includes liability, consent, and fairness. Furthermore, rational distrust among competing institutional world views may be critical in understanding the role of social capital in socioeconomic and technological development. Thus the concept of trust has become a portmanteau, carrying a diverse range of ideas and conditions for sustainable energy systems. The paper concludes with three emphases for decision makers. First, the issue is the energy system, not particular generating technologies. Second, the energy system must be recognized to be as much a social system as it is a technical one. Third. the system requires incorporation of the minimum level of diversity of engineering technologies and social actors to be sustainable. (C) 2009 Published by Elsevier Ltd.