The aim of the thesis was to define and explain the basic concepts of the tax system, the legal basis and provide an overview of legal regulations and the basic types of taxes. The theoretical part is focused on defining the very nature and importance of taxes and the related conceptual terms. The operational part was to analyze whether the introduction of flat tax in the Czech Republic and Slovakia brought some benefit or not. To achieve these objectives in their work, I will use methods of analysis and synthesis, comparison, and statistical regression analysis. Thesis consists of three chapters. In the first chapter I focus on the definition of basic concepts related to the issue of a flat tax. The second chapter is devoted to tax reform ...