What impact does the European Union (EU) have on the development of the rule of law in South Eastern Europe (SEE)? The author of this article argues and shows that the EU has: 1) a positively reinforcing (healthy) effect with regard to judicial capacity and substantive legality, i.e. the alignment of domestic legislation with international standards, and 2) a negatively reinforcing (pathological) effect with regard to judicial impartiality and formal legality (the inner morality of law). The author explains the pathological impact of EU-driven rule of law reforms by referring to the EU’s deficient reform approach and to unfavorable domestic conditions, which in their interplay reinforce certain reform pathologies (legal instability, incoherence, politicization) that undermine the rule of law. The main argument is supported by a mixed method study. A quantitative indicator-based analysis measures rule of law development across four key dimensions on the basis of a variety of data (e.g. survey-based indicators, CEPEJ data, and a unique dataset on legislative output). Additionally, the author draws on a number of qualitative interviews that he conducted with magistrates from SEE and representatives from the EU, the European Court of Human Rights, and the Council of Europe. The author concludes from these findings that external rule of law promotion in weak rule of law countries is not transformative, but rather reinforces systemic deficiencies that undermine the rule of law.