The discussion of illegality as a ground continues in this chapter. The first concern here is to explain the importance to decision-making of the fettering principle since deciding by reference to a policy has the capacity to undermine statutory discretion. Sections are included on legitimate expectation relating to circulars and policy, estoppel and the fettering of discretion, fettering discretion by contract, and fettering discretion by wrongful delegation. In the final sections the chapter considers technical aspects of illegality including: the distinction between errors of law and errors of fact, the emergence of error of precedent fact, the ‘no evidence’ principle, and challenges to the validity of delegated legislation. The case law in this area is analysed in some detail.