The experience of international organizations in the use of existing tools for evaluating the effectiveness of the tax system and tax administration is studied. The most important indicators of the efficiency of tax administration are indicated and characterized. The reasons for the change in the amount of tax revenues to state funds, which depend not only on the activities of the tax service, but also on other factors, have been identified. There are four groups of countries according to the indicators of the tax gap, which are proposed by IMF experts: for each of them the size of the tax gap is set, which is evidence of the effectiveness of tax administration. The analysis is performed and the categories of countries in keeping with the indicators of the tax gap are characterized. Methods of measuring the tax gap and the effectiveness of tax offices in foreign countries have been studied and characterized, taking into account world practice. It is proposed in close work with taxpayers to use different approaches to obtain positive results from the efficiency of the tax system through improving legislative procedures, struggle with tax evasion, deepening work that is customer-oriented.