The extent of harm due to patient safety problems varies across studies. This systematic review sought to estimate the prevalence of preventable harm in medical care overall. Researchers synthesized data from 70 studies and estimated that 6% of patients receiving medical care experience preventable harm. Harm related to medications, diagnosis, health care–associated infections, and procedures accounted for significant proportions of preventable harm. The authors conclude that focusing on evidenced-based strategies to address preventable patient harm would improve health care quality and subsequently reduce costs. A related editorial calls for improving measurement of preventable harm. Another editorial spotlights the importance of understanding the causes of preventable harm in health care.