Building energy simulation is playing an increasingly important role in the development and implementation of building energy codes and standards in the United States. This trend parallels a progression over the past 15 years from the use of largely prescriptive methods for encouraging energy-efficient building design to reliance on more performanceoriented approaches. A multiyear research project is currently under way to develop a methodology on which to base future energy performance standards for the design of new commercial buildings. This research effort, the Whole-Building Energy Targets project, is sponsored by the U.S. Department of Energy and is being conducted with the assistance of leading buildings-industry associations in the United States. The project, the technical approach under development (in particular its use of economic optimization), and additional potential applications of the methodology are described in this paper.