This monograph elucidates common legal principles underlying the use of juridical powers. It addresses both public law and private law, and examines both the common law and the civil law. It aims to provide a theory of how Western law regulates the situations in which we hold legal powers, not for ourselves, but for and on behalf of others. It does this by elucidating the justificatory principles that are attracted in those situations. These principles include that other-regarding powers can only properly be used for the purposes for which they were granted; that they should not be used when the holder is in a conflict of self-interest and duty, or a conflict of duty and duty; and that the holder is presumptively accountable for any profits extracted from the other-regarding role. These principles stand behind the detailed legal rules that govern these relationships in multiple legal systems and in multiple public and private settings. In private law this includes the powers of trustees, corporate directors, agents and mandataries; in public law it includes all powers held for public purposes, whether they be held by the Prime Minister, by a police officer, or by a judge.Lionel Smith, BSc (Toronto), LLB (Western Ontario), LLM (Cantab), DPhil, MA, DCL (Oxon), LLB (Montreal) taught at the Universities of Alberta and Oxford before joining McGill University in 2000. He has been Sir William C Macdonald Professor of Law at McGill University since 2013. Previously he was Director of the Paul-Andre Crepeau Centre for Private and Comparative Law and James McGill Professor. He is a Titular Member of the International Academy of Comparative Law and member of the American Law Institute, the European Law Institute, and the International Academy of Estate and Trust Law, and is a non-practising member of the Bar of Alberta. From 2022 he will be the Downing Professor of the Laws of England at Cambridge University and a fellow of Gonville and Caius College.