Nowadays, the office of public prosecutor is the commonly accepted legal institution in the Western legal culture. Its existence is understood as something taken for granted. This was different in the Middle Ages. At first, the criminal trial proceedings were not distinguished from the civil ones, and therefore they were conducted on the basis of the same fundamental principles. There was no public authority engaged in instituting the criminal trial. The latter had to be instituted by private individuals who were the injured parties. This had an impact on the forming of the concept of crime which was not viewed as an offence against the society or the State but against the injured individual. The paper is concerned with the medieval Kingdom of Hungary and discusses the development of State structures, criminal substitutive law and the criminal procedure.