The principle of nullum crimen, nulla poena sine lege is the most important principle of Korean criminal law. It is commonly understood as being composed of four sub-principles: lex certa, lex praevia, lex stricta and lex scripta. It was given lip service under the authoritarian regimes, but the military rulers never felt restricted by those in exercising the state power. After democratization, however, this principle has been actively pursued. Many legal scholars and civic organizations have argued that the task of criminal law is to protect citizens from the abuse of state authority as well as tofight against crimes. The legitimacy of a number of criminal law provisions in violation of this principle has been challenged in academic circles and in the courts. The principle of nullum crimen, nulla poena sine lege will play a crucial role in controlling and deterring the abuse of state authority in Korean society and freeing the criminal law from the legislation of moral norms.