The aim of this paper was to investigate public understanding of hydrogen energy using a particular social-psychological theory, namely, the theory of social representations to explore how processes of understanding generated lay knowledge of hydrogen energy. Using a free association method for data collection and multidimensional scaling for analysis, the results enabled the identification of themes in the data such as energy, environment, community, science and technology, and people and place, around which understanding was based Processes of representation, such as anchoring to pre-existing knowledge, were seen as essential in guiding understanding. The results indicated that there were diverse influences involved in understanding and, although risk perception of hydrogen was acknowledged, community concerns were seen to override any negative effect of focussing on risk. The role of emotion in decision-making was highlighted as positive emotional responses to the Promoting Unst's Renewable Energy (PURE), a local hydrogen storage project, resulted in hydrogen energy generally being positively evaluated despite acknowledged risks posed by hydrogen such as its explosiveness and flammability. Recommendations for policy include recognising that the combination of expert and lay knowledge plays an important role in public acceptance or rejection of hydrogen energy (c) 2009 Elsevier Ltd All rights reserved.