The author describes two crucial periods in Slovak legal history from the perspective of the principle of legal security in criminal law. Firstly, the 19th-century Criminal Code, known as the Csemegi Code, is at the center of attention as it ended the centuries-long legal insecurity resulting from the lack of criminal law codification. The author analyses its leading principles as the safeguard of legal security. In contrast, the second part of the article focuses on the second codification of criminal law in Slovak legal history, the Criminal Code of 1950. The author demonstrates how the totalitarian regime destroyed the principle of legal security in criminal law in Slovakia, only slowly rebuilding it after the death of Joseph Stalin through amendments in 1956 and new legislation in 1961. The current Slovak Criminal Code and Criminal Procedure Code have been in effect since 2006. In conclusion, the author draws attention to how the latest changes in criminal law codes caused turbulences and worries over the principle of legal security in Slovakia.