The aim of the paper is to expound the issue of criminal liability of a co-perpetrator of an offence (for instance, an aider or abettor), particularly in the case where the act, permissible by the legislation in force in the country where the chief perpetrator operated, is prohibited under penalty in the Republic of Poland. In this connection, matters such as state sovereignty, the functions of criminal law and the scope of protection of legal goods are discussed. In the first part of the article, provisions of the Polish Criminal Code that regulate co-perpetration are presented. Furthermore, a dogmatic analysis of the term “prohibited act” is attempted, followed by a corollary that for the sake of the protection of legal goods it is required that Polish criminal law react to the acts of the co-perpetrator even where the perpetrator acted within the bounds of the overseas law. The legality of the act committed by the perpetrator abroad is of no consequence as against the assessment of the criminal character of the co-perpetrator’s acts insofar as they were criminal pursuant to the law of the Republic of Poland.