This paper considers contractual discretions in the construction context. It considers the types of discretions conferred on the principal or contractor and how they might be constrained by express or implied terms. It examines this in light of the Supreme Court’s leading decision in Braganza v BP Shipping Ltd,2 which, first, provides guidance on how the scope of the implied duty varies according to the terms and context of the Contract, and second, draws an analogy between contractual discretions and the administrative law review of statutory powers. Finally, this paper concludes by arguing that imposing “good faith” restrictions on contractual powers is likely to promote fair contracting within the construction industry, without adding unmanageable uncertainty.