The project is based on the premise that the principle of legality is currently undergoing a continuous process of erosion in the Spanish criminal justice system as a result of various dynamics of transformation in relation to the public institutions closest to the guarantees that derive from its meaning, that is, the legislature and the judiciary. However, the application of the law in practice is still largely carried out as if the old paradigm of criminal law as ultima ratio and lex certa were still in force, with the courts as mere interpreters of the law and fundamental rights acting only as limits to punitive power. This situation of dissonance makes it difficult to identify the problems and to assess their magnitude, as well as to reflect on whether it is necessary to rethink some of the traditional categories and rules of operation and to put forward, if necessary, proposals for improvement.