Higher patient-to-nurse ratios have consistently been associated with adverse effects on patient safety and inpatient mortality, but the mechanism driving this relationship remains unclear. Missed nursing care—a type of error of omission in which required care elements are not completed—is relatively common on inpatient wards. This study sought to investigate the relationship between nurse staffing, missed nursing care, and patient safety in England. Nurses frequently reported leaving care undone, and missed nursing care episodes were strongly associated with higher numbers of patients per nurse and lower safety culture ratings. The authors argue that the frequency of missed care episodes should be used to measure nursing quality and that improving the overall work environment for nurses should be a patient safety priority. A preventable death due in part to inadequate nurse staffing is discussed in an AHRQ WebM&M commentary, and the critical role nurses play in ensuring patient safety is explored in a Patient Safety Primer.