In this article, the author discusses the principal purpose test (PPT) included in article 29 of the OECD and UN Models (2017), arguing in particular that while the PPT certainly permits a purposive interpretation, it may not be used to build into tax treaty law additional requirements that were never intended. Finally, the author concludes by looking at the other side of the coin and wonders whether a comparable “PPT” should not regulate the performance by States of their treaty obligations.