jats:titleAbstract</jats:title>jats:pSection I shows that the idea, founded on the “principle of legality”, that all legal flaws make a decision void as a matter of law is erroneous. Infringing a legal requirement may not affect validity, or may make a decision only voidable. Section II shows the significance of distinctions between various stages of decision-making processes, and between different types of issues for judges, and argues that case law shows that seven guiding principles operate alongside the “principle of legality”. Section III concludes that these common-law principles reflect professional practice and provide a realistic basis for predictable, normatively legitimate administrative law.</jats:p>