From a sociotechnical perspective, improving energy efficiency and reducing energy demand are understood as social processes with complex interactions between multiple actors (firms, researchers, policymakers, consumers) who develop strategies, make investments, learn, open up new markets and develop new routines. A sociotechnical perspective views energy services as being provided through large-scale, capital-intensive and long-lived infrastructures that co-evolve with technologies, institutions, skills, knowledge and behaviours to create broader ‘sociotechnical systems’. This chapter develops a rationale for the sociotechnical approach, describes its general characteristics and identifies some core research debates. It elaborates upon three research themes – emergence, diffusion and impact – and identifies key issues and debates within each.