The article emphasizes the importance of legal security in criminal law. The proposed definition of legal security – according to which legal security is a state achieved by law established in general, and in particular by means of criminal law, in which human life’s goods and interests are protected in the most comprehensive and effective possible manner – harmonizes properly with the most important functions of criminal law, namely with a protective function and a guarantee function. The proposed understanding of legal security may be helpful for clarifying numerous issues related to the implementation of both the protective function and the guarantee function of criminal law. Although legal security is a particularly important value in criminal law, in the hitherto achievements of the doctrine and in the jurisprudence of criminal law, this value is sparsely the subject of considerations. The article indicates the necessity and possibility of further detailed research in the field of implementing the idea of legal security in criminal law.