The article discusses the relationship between tax rules and the 1988 Federal Constitution, focusing on the granting of tax benefits. It highlights the importance of understanding the role of the Constitution in granting these benefits to guarantee their legality and effectiveness, without compromising financial stability and tax equity. Divided into two parts, the first analyzes the Financial Constitution and its role in the State’s financial order, especially in tax exemptions. The second part explores the principle of legality of tax benefits and their constitutional implications, considering fiscal sustainability and tax equality.