Commentary on Ruling no. 171/2017 – meaning and scope of the constitutional prohibition of retroactivity in tax law

Abstract

This text focuses on Ruling 171/2017 of the Portuguese Constitutional Court, which discusses the unconstitutionality of a retroactive tax provision. This ruling follows previous case law on this issue, both in terms of the outcome – unconstitutionality – and the constitutional parameter on which it is based – the prohibition of tax retroactivity in Article 103(3). However, Ruling 171/2017 departs completely from previous rulings in some underlying premises. These premises will be critically analyzed, comparing this ruling with previous case law on this issue and determining its impact on subsequent rulings.