This chapter begins with a discussion of sources of power, including statutory powers and duties, the royal prerogative, corporate and contractual powers, and non-legal and abnormal powers. It then describes the conditions imposed on powers conferred on public authorities; the power to revoke or modify the decisions or orders of an administrative authority or tribunal; situations in which the court will hold administrative decisions to be irrevocable; the principle of estoppel; waiver and consent; and the role of res judicata in administrative law.