A serious shortage of hospital nurses in the U.S., evident in the past decade, is expected to continue and worsen in the next 15 years. Increasingly, the public and the health professions are acknowledging that nurse understaffing represents a serious threat to patient safety in U.S. hospitals. Although anecdotal evidence has linked patient deaths to inadequate nurse staffing, the numbers and kinds of nurses needed for patient safety is unknown. This Issue Brief highlights two studies that clarify the impact of nurse staffing levels on surgical patient outcomes, and examine the effect of nurses' experience and educational level on patient mortality in the 30 days after a surgical admission.