Long hours are an accepted component of resident education, yet data suggest they contribute to fatigue that may compromise patient safety. A systematic review confirms that limiting duty hours increases residents' hours of sleep and improves objective measures of alertness. Most studies of operative experience for surgical residents found no effect, and there is evidence of a limited positive effect on residents' mood. We find a mixed effect on patient safety, although problems with supervision, rather than the limits, may be responsible or contibute; evidence of reduced continuity of care and reduced continuity in residents' clinical education; and evidence that increased workload under the limits has a negative effect on patient and resident outcomes. We highlight specific areas for research and offer recommendations for national policy.