Over the course of the nineteenth and twentieth centuries, the United States developed awealthy society on the basis of cheap and abundant fossil fuel energy. As fossil fuelshave become ecologically and economically expensive in the twenty-first century,America has shown mixed progress in transitioning to a more sustainable energy system.From 2000 to 2006, energy and carbon intensity of GDP continued favorable long-termtrends of decline. Energy end-use efficiency also continued to improve; for example, per-capita electricity use was 12.76 MWh per person per year in 2000 and again in 2006,despite 16 percent GDP growth over that period. Environmental costs of U.S. energyproduction and consumption have also been reduced, as illustrated in air qualityimprovements. However, increased fossil fuel consumption, stagnant efficiencystandards, and expanding corn-based ethanol production have moved the energy systemin the opposite direction, toward a less sustainable energy system.This chapter reviews energy system developments between 2000 and 2006 and presentspolicy recommendations to move the United States toward a more sustainable energysystem.