How do we understand when things don't go as planned in healthcare and how do we learn from the vast majority of times when things work out just fine? The science of patient safety deals with preventing harm in healthcare settings. We explore the core ideas behind three views – Safety I, II, and III – complementary approaches that tell us when we should find and fix patient safety problems, learn from what went right, and identify and reduce hazards. Carsten Engel tells us about Denmark's national patient safety incident reporting systems, one of the oldest such databases in the world.