Modernity is complex. Contemporary societies, economies, and technologies co-evolve into systems that are fast-changing and display increased complexity. Complex systems have multiple variables, which makes them hard to describe, and even harder to predict. As law and the State cannot be decoupled from these phenomena, it is only natural for legal systems to also become complex. At the Working Group on Law, Rationalism and Complexity, we have a shared interest in relying on rationalism in the analysis of today’s complex legal systems. We understand that legal systems are multifactorial and co-constructed by environmental, political, economic, social, and cultural factors. We aim to use rationalism as a tool to study complex legal phenomena. By rationalism, we mean an approach that aims to track truth, independently of the characteristics or opinions of the observer. The hallmarks of rationalism are a commitment to logical reasoning, empirical fact-finding, and analytical thinking. A rational study of complex legal systems is aware of the limits of established paradigms (including rationalism itself) and has an open-minded attitude toward reasoned heterodoxy for the description of reality. Accordingly, promoting value-free and apolitical empiricism in applying scientific method(s) constitutes a complementary approach in the study of legal complexity. By adopting a multi-factorial analysis of legal systems, we can contribute to a more accurate understanding of the complex contemporary legal landscapes. We also consider that this approach will help mitigate our own epistemic biases. The working plan is to develop knowledge for at least the first six to twelve months of operation of the Working Group. Once the foundations of a rational study of complex legal systems are in place, we aim to (i) consolidate the knowledge gathered in a memorandum; and (ii) outline further directions for the Working Group discussions and presentations.