The purpose of this article is to analyze the 2023 decisions of the Supreme Court (‘SC’) on tax procedures and income taxation. The main ideas of this article are as follows. First, the SC 2021Du46742 ruling (November 2, 2023) and others nullified tax impositions by acknowledging procedural defects in tax audits. Such conclusions are righteous. It is necessary for the National Tax Service to make efforts to comply with procedural rules in tax audits. Second, the SC 2021Du37748 ruling (November 2, 2023) and the SC 2022Du45968 ruling (December 7, 2023) cleared the scope of the procedural issues of the Request for the Review on Appropriateness of Tax Imposition (‘RATI’). These decisions are based on the due process of law in the area of taxation and will increase the availability of the RATI. Third, according to the SC 2020Du48215 ruling, several value amounts prescribed by Article 178-3 of the Enforcement Decree of the Personal Income Tax Act are typical examples of overseas assets’ market prices. In light of the concept of market prices and the SC precedents, this judgment is correct. Fourth, the SC 2018Du33005 ruling (May 18, 2023) and others provided specific factors for determining economical reasonableness as to trademark transactions between related parties. The position of the SC is based on the transfer pricing regime. It is necessary to discuss how to set up evaluation rules regarding trademark royalties. Fifth, the issue of the SC 2020Du37857 ruling (November 30, 2023) is whether the substance-over-form principle may be applied to a joint venture transaction between a Korean corporation and a foreign company. The SC denied the application of the substance-over-form principle by regarding the deemed dividend received by the Korean corporation as the distribution of the joint venture’s business performance. This ruling is an important precedent for determining tax avoidance intents and business purposes.