Energy efficiency may be the farthest-reaching, least-polluting, and fastest-growing energy success story of the last 50 years, but this success is also invisible, misunderstood, and in serious danger of missing out on needed future investments. That’s the assessment of the American Council for an Energy-Efficient Economy (ACEEE) after its attempt to quantify the overall impact of the hidden U.S. energy efficiency boom. The resulting report shows that U.S. energy consumption (as measured per dollar of economic output) will have been slashed by the end of 2008 to half of what it was in 1970, from 18,000 Btus to about 8,900 Btus.