The cohesion of the tax system involves harmonization, logic and transparency of legal regulations forming the tax system. Each tax system operating properly has to be coherent and different kinds of taxes have to be harmonious without any possibility of contradiction between them. The aim of this paper is to characterize the essence of the tax system cohesion in the interior and exterior context. The analysis is devoted to the cohesion of the taxes forming the Polish tax system and the necessity of harmonizing a given tax construction with other constructions as well as legislation in general and administrative and civil law solutions in particular. What is more, the paper analyses the range of harmony between the Polish tax law and international regulations as far as the avoidance of double taxation is concerned.