In federal systems the courts are accorded the important role of policing the boundaries of the constitutionally specified powers given to the legislature and the executive. The devolution statutes enacted by the UK parliament have created a semi-federal system, in which the courts increasingly have been called upon to adjudicate on whether sub-national legislative bodies have acted ultra vires. Following a comparative overview of the theory and practice of court adjudication in overseas systems of multi-level governance, the leading cases of the British Supreme Court and the House of Lords are considered. It is proposed that Britain—almost by default—has become a semi-federal court somewhat akin to the US Supreme Court and the German Bundesverfassungsgericht.