The aim of the Doctoral Thesis is to develop tools for improving the tax system in Latvia and increasing its reliability, considering the multidirectional interests of the state and business, for the following integrated optimization. In the first chapter, the issues of tax policy theory, summarizing the theoretical aspects of tax evolution and tax application, systematizing tax policy criteria and conducted research related to the application of the supply economics theory and the A. B. Laffer curve as state-oriented criteria for the optimization of national tax policy are investigated. In the second chapter a study was conducted on the impact of the European Union’s tax policy on the development of Latvia’s tax policy from 1990 to 2021 and the EU’s tax requirements, which are limiting the optimization criteria of state-oriented tax policy optimization. In the third chapter, the process of optimization of taxation is analysed in two aspects: state-oriented and business-oriented, the concept of reliability of the tax system is introduced. A tax prism method is developed, and a variant optimization method is proposed to solve the problem of determining a rational combination in the “equity – efficiency” system. The tax system is transformed into a composite system, the classical and quantum approaches are considered. In the fourth chapter, using the tools developed in Chapter 3, the results of assessing the reliability of the Latvian tax system after the reforms of 2018 and 2020 are presented, as well as the impact of external socio-economic indicators on total tax collections (including social payments). The application of the tax prisms is demonstrated. Time series of monthly tax revenues to the Latvian budget, starting from 2016, are constructed and analysed using ARIMA and SARIMA models. For the implementation of variant optimization, combined diagrams are built, the results of processing a survey of experts are given. Based on the analysis of the transport tax in force (including other tax factors, impacting the cost of vehicle usage) in Latvia, and its scenario analysis is carried out. The content of the work (introduction, four chapters, conclusions and proposals) is presented on 164 pages (excluding annexes), illustrated with 43 figures and 9 tables, and has 26 formulas. The bibliography includes 264 references. 11 annexes are attached to the work.