The personal scope of customs debt and excise taxation differ. It is clearly noticeable, however, that legislators – in the spheres of customs and excise duties – tend to eliminate the possibility of untaxed consumption of goods. In order to achieve this aim, the legislators impose duties on the holders of untaxed goods. The author of this article analyses the relevance of knowledge of the holders for the personal scope of respectively customs and excise duties in the legal system of the European Union (EU) with particular emphasis placed on Poland and undertakes an attempt to assess the rules from the perspective of the principle of legal certainty.