This report helps define a vision for achieving energy efficiency in U.S. data centers. Although the growing energy use of servers and data centers makes this a challenging goal, there are large opportunities for savings. These savings will not be easy to achieve, given the barriers outlined in this report, but there are many policies available to overcome the barriers. Realizing these efficiency gains will take coordination and collaboration among many stakeholders: the government, the IT industry, data center operators, electric utilities, and others. The outlook for efficiency gains is encouraging, though, because industry is very engaged with these issues and is working with customers who are demanding solutions to the growing energy use in data centers. Federal initiatives should build on these efforts and partner in ways that develop objective, credible information, benchmarks, metrics, and industry standards. Finally, as a significant operator of data centers itself, the federal government can help facilitate change by changing the way it designs and operates its own facilities.