Two things', wrote Kant, 'fill the mind with ever new and increasing admiration and awe: the starry heavens above and the moral law within'. Many would argue that since Kant's day, the study of the starry heavens has advanced while ethics has stagnated, and in particular that Kant's ethics offers an empty formalism that tells us nothing about how we should live. In Acting on Principle Onora O'Neill shows that Kantian ethics has practical as well as philosophical importance. First published in 1975, the book is regarded as a classic account and defence of the Kantian ethical position. It addresses Kant's account of reasoning about action, in particular his controversial claim that the Categorical Imperative guides action and is basic to ethics and justice. This second edition offers a substantial new introduction and updated bibliography, and will be valuable for a wide readership in Kant studies and those studying ethics.