The built environment accounts for approximately forty percent of the total energy consumption in developed countries. Because buildings have a long life, the greatest opportunity for energy reduction in the built environment will come from energy conservation in the existing building stock. An overview of the policy challenges presented by the built environment, with an emphasis on existing facilities, is accompanied by a discussion of specific technologies that may have the potential to reduce energy use. To illustrate the degree of complexity associated with shifts to new technologies, lighting, a major consumer of electric energy in the built environment, is described in an expanded narrative by Kevin Kelly. The chapter concludes with a discussion of recent policy schemes that have been employed in the United States and the European Union to promote energy conservation in the built environment. Both voluntary and public policy programs are included, along with an examination of resultant successes and failures. Based on this discussion, a series of recommendations and opportunities for future solutions is provided for each of the challenge areas presented earlier in the chapter.