In my PhD project ”Who Decides Over EU-legislation? A New Assessment of the Co-legislators’ Relative Influence”, I study legislative negotiations within the EU. The primary goal is to measure the relative influence of the European Parliament and the Council of the European Union over acts that are negotiated in so-called “informal trilogues”. The institutional balance of influence remains unclear in prior research, but is important to understand considering the central role of EU legislation in the member states. By conducting interviews with practitioners both within the EU institutions and in the public administrations of the member states, I hope to gain a better understanding of how trilogues work in practice. The interviews will contribute to an in-depth description of the legislative process and to the design of a large-N study of the co-legislators’ relative influence.