This chapter considers the grounds on which public decisions may be challenged before the courts. It begins with an overview of two cases—Associated Provincial Picture Houses Ltd v Wednesbury Corpn (1948) and Council of Civil Service Unions v Minister for the Civil Service (1985). The importance of these two cases is their distillation of the general principles. The discussion then covers the different grounds for judicial review: illegality, relevant/irrelevant considerations, fiduciary duty, fettering of a discretion, improper purpose, bad faith, irrationality, proportionality, procedural impropriety, natural justice, legitimate expectations, the right to a fair hearing, reasons, and the rule against bias. It is noted that principles often overlap, so that a challenge to a public law decision may be based on different principles.