This paper describes the approach to developing transition pathways for a low carbon electricity system in the UK, being pursued in a major new inter-disciplinary research project. The project aims (a) to learn from past transitions to help explore future transitions and what might enable or avoid them; (b) to design and evaluate transition pathways towards alternative socio-technical energy systems and infrastructures for a low carbon future; and (c) to understand and, where appropriate, model the changing roles, influences and opportunities of large and small 'actors' in the dynamics of transitions. The paper describes the approach, which builds on the work of Dutch researchers on transitions and transition management using a multi-level framework of niches, socio-technical regime and landscape, as well as on other parts of the innovation systems literature. It also describes its application to several outline transition pathways to a low carbon energy system in the UK. The pathways embrace both the evolution of the physical and institutional infrastructure changes and the roles of both large actors, e.g. multinational energy supply and distribution companies, national governments, major investors, and small actors, e.g. households, innovators and entrepreneurs.