Constitutional courts are at the core of political systems. They have an impact on citizens’ rights and on political processes such as elections and legislative procedures. These institutions execute their power exclusively, since no other court or body can engage in constitutional review. In our project, we are particularly interested in answering the question, to which extent European Constitutional Courts including national Constitutional Courts in the European Union (EU), the Court of Justice of the European Union (CJEU) and the European Court of Human Rights (ECtHR) base their decisions exclusively on the legal text of their constitution and to what degree they are seeking guidance by external institutions.