Acknowledging the inevitable connection between fatigue and patient safety, Gaba and Howard examine the policies for work hours for clinicians-in-training in the United States. At the time of publication, regulations varied by specialty and, despite policies limiting work hours, many residency-training programs did not comply with these limited standards. On the heels of increasing proposals to regulate work hours set forth by house staff, lobbying organizations, and Congress, the Accreditation Council for Graduate Medical Education (ACGME) announced new requirements for limited work hours beginning July 2003 for all residency programs. The authors discuss the likely consequences of these limitations, including decreased work force, increased hand-offs, and restrictions on moonlighting. Specific challenges, including behavioral and cultural shifts as well as potential economic impact, are discussed.