All over the world, in all democratic States, independently of having a legal system based on the common law or on the civil law principles, the courts – special constitutional courts, supreme courts or ordinary courts – have the power to decide and declare the unconstitutionality of legislation or of other State acts when a particular statute violates the text of the Constitution or of its constitutional principles. This power of the courts is the consequence of the consolidation in contem-porary constitutionalism of three fundamental principles of law: first, the existence of a written or unwritten constitution or of a fundamental law, conceived as a superior law with clear supremacy over all other statutes; second, the “rigid” character of such constitution or fundamental law, which implies that the amendments or reforms that may be introduced can only be put into practice by means of a particular and special constituent or legislative process, preventing the ordinary legislator from doing so; and third, the establishment in that same written or unwritten and rigid constitution or fundamental law, of the judicial means for guaranteeing its supremacy, over all other state acts, including legislative acts. Accordingly, in democratic systems subjected to such principles, the courts have the power to refuse to enforce a statute when deemed to be contrary to the Constitu-tion, considering it null or void, through what is known as the diffuse system of judicial review; and in many cases, they even have the power to annul the said unconstitutional law, through what is known as the concentrated system of judicial review. The former, is the system created more than two hundred years ago by the Supreme Court of the United States, and that so deeply characterizes the North American Constitutional system. The latter system, has been adopted in consti-tutional systems in which the judicial power of judicial review has been generally assigned to the Supreme Court or to one special Constitutional Court, as is the case, for example, of many countries in Europe and in Latin America. This concentrated system of judicial review, although established in many Latin American countries since the 19th century, was only effectively developed particularly in the world after World War II following the studies of Hans Kelsen. Of course, during the past thirty years many changes have occurred in the world on these matters of Judicial Review, in particularly in Europe and specifically in the United Kingdom, where these Lectures were delivered. Nonetheless, I have decided to publish them hereto in its integrality, as they were: the written work of a law professor made as a consequence of his research for the preparation of his lectures, not pretending to be anything else, but the academic testimony of the state of the subject of judicial review in the world in 1985-1986. Allan R. Brewer–Carías