Résumé : The thesis evaluates how effective the European Court of Human Rights and the Court of Justice of the EU have been in promoting the human rights of vulnerable migrants. It thereby pursues two objectives. Firstly, it questions legal scholarship that has identified certain rulings of the two courts as vital for migrant rights but which have not analysed their impact empirically. Secondly, it makes a methodological contribution to the evaluation of the effectiveness of international courts by proposing (and applying) an 'issue-based' methodology which assesses judicial bodies for their ability to resolve specific social and political problem. For these purposes, eight carefully selected 'key cases' are analysed in terms of three effectiveness types, namely law development, case-specific, and strategic effectiveness. The empirical materials used include interviews with 28 persons who were directly or indirectly involved in the selected key cases. Legal and empirical analyses show varying and complex results for the different cases, with some general trends emerging. Firstly, the case law of the courts is characterised by a significant inconsistency, resulting in a 'dilemmatic adjudication' that diminishes the impact of even rights-affirming judgments. Secondly, the 'case-specific' impact on the persons or the countries concerned remains contingent as governments and domestic courts manage to contain the ruling. Lastly, lawyers and NGOs increasingly use the courts in a strategic way, which can elevate impact on policy. Yet, this will depend on the resources invested, raising the question whether such strategies are the most efficient way to promote migrant rights.