This thesis introduces a new analytical paradigm to research public international law. It employs complex adaptive systems (CAS) theory to analyze the interacting and evolving network of international rules and regimes. It focuses on the systemic nature of international law, its structure and the patterns of interaction that arise between its elements. To explore different micro- and macro-level regularities in the interaction of norms and regimes this thesis uses international trade, investment and tax as a case study. The thesis’ core argument is that international law and its regimes can be conceptualized as nested CAS whose elements and meta-elements organize in the absence of central control. This conceptualization can help answer questions as to the system’s future development and how its evolution can be directed. The thesis’ first part introduces core concepts as well as the complexity framework whose application to legal systems is illustrated. The second part describes the tax, trade and investment regimes from the CAS perspective to generate novel insights as to their rules’ nature, function and interaction. It then moves to the level of the entire system of public international law and describes the functional relationships and adaptive processes that exist within and between legal regimes. The third part focuses on the prescriptive and predictive aspects of the CAS framework. It explores what consequences the increasing specialization, diversity and interconnected nature of international law entail for its future evolution. Moreover, lever points for effecting change and steering the CAS of public international law are illustrated.