Energy use in buildings is influenced by a variety of factors in complex ways. Historically, in the UK the efficiency of energy use in buildings has not been a major consideration in their design. Now policy concern with climate change is changing this, because buildings have come to be perceived as the locus of energy use with the highest cost-effective energy savings potential. In the UK, the energy efficiency of the building stock is rather low. The paper focuses largely on energy use in the UK's existing building stock and the two main European Union Directives which affect it: the Energy Performance in Buildings Directive and the Energy Services Directive. The Directives are complex, and there are a number of supporting programmes set up by the European Commission to aid their implementation. Even so, they have been implemented in differing ways in different European countries,and implementation remains patchy. The Directives have the potential to be a major influence on the evolution of the UK's built environment, but their effect will depend on the details of the Directives implementation and enforcement, many of which are not yet clear.