This advanced seminar explores the application of complexity science to understand multilateralism. Students will work in groups on their own project and apply suitable computational data science methods (e.g., network analysis, natural language processing, time series analysis, machine learning) to understand topics related to multilateral cooperation. In their projects, students will collect, preprocess and analyse real-world data on multilateralism by using one or several programming languages (Python and R) with the help of a large language model. The seminar integrates perspectives from international legal theory, complexity science, and systems thinking to develop a hands-on approach that applies data science to the study of international relations.