This paper attempts to analyze and interpret the principles of direct and indirect taxation. Initially, a theoretical approach to the general concepts and principles governing tax law is presented. It is followed by a presentation of the Greek tax and financial system. The key points demonstrating the harmonization of tax legislation, in accordance with the OECD (Organization for Economic Cooperation and Development) guidelines so far are highlighted. Any future reform of the Greek national tax system should incorporate the fundamental principles of national tax policy. Additionally, it must be in line with OECD principles and guidelines. The further implementation of the Organization's recommendations will shift the balance between direct and indirect taxation, leading to strengthening the first and reducing the second. Over the last decades, the ratio of direct to indirect taxes has been on the increase, particularly in developed countries. The main reason is that direct taxation is socially just and effective. Direct taxation is attributed by the liable person, while indirect taxation is transferred according to the ability of each person to react to the relevant market. The OECD supports, for member countries, the strengthening of direct taxation, which takes into account income and other social criteria and satisfies both the criterion of efficiency and the criterion of fair distribution of economic burdens. On the contrary, indirect taxation is reduced to inversely progressive taxation in terms of the income of consumers and is not consistent with the OECD principles of socially just and effective taxation in economically developed countries. The taxation of the citizens in general follows the principle of the citizens contributing to the public burdens according to their powers, for there is nothing more unjust than to treat unequal men as equals. From this proportional treatment of the taxing units, the constitutional principle of equality is also satisfied by reflection.