Motivating individuals to decrease the environmental impact of their lifestyles could play an important role in reducing energy use and meeting carbon reduction commitments in developed countries. Few approaches which encourage voluntary changes in behaviour result in substantial reductions in energy use, however, particularly over the longer term. An exception to this general trend is small-group interventions which use group participation and which target collections of behaviours including energy use. Through a critical examination of published data this paper considers the energy and carbon emission reductions achieved by such initiatives, the durability of those reductions, and the common elements which may contribute to their success. Participants in small-group interventions reduced their energy use and carbon emissions by approximately 20% within a year. There is also some evidence that these reductions were lasting and that participants continued to make changes to their lifestyles after the end of the intervention. The reasonable person model (RPM) is proposed as a useful framework for understanding the success of these small-group interventions. Examination of small-group interventions suggests that they provide settings which are supportive of informational needs, and that this may be important to their success in promoting substantial and durable decreases in energy use.