This article presents an overview of the legal regime provided in the Spanish system of criminal sanctions regarding the control of dangerous sex offenders in the community. It focuses on the introduction, in 2010, of a post-prison safety measure named supervised release. We describe the context of its introduction in the Spanish Criminal Code, considering the influence of societal upheaval concerning dangerous sex offenders in its development, and also the historical and theoretical features of the Spanish system of criminal sanctions. We also analyse the legal framework of supervised release, the existing case law about it and how the legal doctrine has until now assessed this measure. After this analysis, the main aim of this article consists in evaluating the effectiveness and the proportionality of the measure, according to the principle of minimal constraints and the rehabilitative function of the criminal sanctions in Spanish law, stated in Article 25.2 of the Spanish Constitution.