Some errors in health care are inevitable because of human fallibility and system complexity. To improve patient safety we must develop three strategies. First, prevent errors with forcing functions, reducing complexity and providing reminders at the point of care. Second, everyone working in health care should be alert to identify and eliminate latent (potential) errors before patients are harmed. Finally, we must establish defensive barriers that will intercept those errors that still occur and prevent them from causing patient injury. Only in this way can health care fulfill its potential and significantly reduce iatrogenic harm.