BACKGROUND: Unsafe medical care may cause substantial morbidity and mortality globally, despite imprecise estimates of the magnitude of the problem. To better understand the extent and nature of the problem of unsafe care, the WHO World Alliance for Patient Safety commissioned an overview of the world's literature on patient safety research. METHODS: Major patient safety topics were identified through a consultative and investigative process and were categorised into the framework of structure, process and outcomes of unsafe care. Lead experts examined current evidence and identified major knowledge gaps relating to topics in developing, transitional and developed nations. The report was reviewed by internal and external experts and underwent improvements based on the feedback. FINDINGS: Twenty-three major patient safety topics were examined. Much of the evidence of the outcomes of unsafe care is from developed nations, where prevalence studies demonstrate that between 3% and 16% of hospitalised patients suffer harm from medical care. Data from transitional and developing countries also suggest substantial harm from medical care. However, considerable gaps in knowledge about the structural and process factors that underlie unsafe care globally make solutions difficult to identify, especially in resource-poor settings. INTERPRETATION: Harm from medical care appears to pose a substantial burden to the world's population. However, much of the evidence base comes from developed nations. Understanding the scope of and solutions for unsafe care for the rest of the world is a critical component of delivering safe, effective care to all of the world's citizens.