The role of increased energy efficiency in achieving sustainable development is investigated. Specifically, the relations are examined between energy (and energy efficiency) and sustainable development. Also, the linkages are discussed between energy efficiency and environmental impact, and the theoretical and practical limitations on increased energy efficiency are described, particularly as they relate to sustainable development. Finally, a case study is presented, in which the benefits achievable through the cogeneration of electricity and heat are considered for Ontario. It is demonstrated that energy resources and their utilisation are intimately related to sustainable development. It is further demonstrated that, for societies to attain or try to attain sustainable development, much effort must be devoted, not only to discovering sustainable energy resources, but also to increasing the energy efficiencies of processes utilising these resources.