Knowledge aims to fit the world, and action to change it. In this collection of essays, Onora O'Neill explores the relationship between these concepts and shows that principles are not enough for ethical thought or action: we also need to understand how practical judgement identifies ways of enacting them and of changing the way things are. Both ethical and technical judgement are supported, she contends, by bringing to bear multiple considerations, ranging from ethical principles to real-world constraints, and while we will never find practical algorithms - let alone ethical algorithms - that resolve moral and political issues, good practical judgement can bring abstract principles to bear in situations that call for action. Her essays thus challenge claims that all inquiry must use either the empirical methods of scientific inquiry or the interpretive methods of the humanities. They will appeal to a range of readers in moral and political philosophy.