The association between lower patient-to-nurse ratios and inpatient mortality has been demonstrated in classic studies, providing an impetus for laws in 15 states that mandate the maximum number of patients per nurse. This study explores the mechanism by which reducing patient-to-nurse ratios improves outcomes. Using a large database of patient discharges and nurse surveys from 665 hospitals in 4 states, the authors found that decreasing the number of patients per nurse improved mortality and failure to rescue predominantly in hospitals rated as having a good work environment. Hospitals with a poor work environment derived no benefit from reducing patient-to-nurse ratios. The critical role that nurses play in ensuring patient safety is discussed further in this Patient Safety Primer.