The laws adopted by Poland in 2015–2018 fundamentally changed the structures of the Constitutional Tribunal, the National Council of the Judiciary and the Supreme Court. The purpose of this article is to critically analyse these changes in regard to the principle of the rule of law, with a focus on legal certainty and the independence of the judiciary. There are analogies between the changes to the judiciary introduced by Poland and those by Hungary which resulted in triggering the Article 7 procedures against both of these countries. There is no universal definition of the rule of law within the European Union, and therefore there are huge ambiguities as to what constitutes a rule of law violation under Article 2 of the Treaty on the European Union. The outcomes of this case study suggest that the formal concept of the rule of law is an insufficient tool to protect constitutional values and thus the substantive concept should be applied.