This thesis analyzes the problem of illicit trade regulation from international and WTO law perspectives. Many authors have described illicit trade as a global threat to states' development and their citizens' well-being. Despite the magnitude of this threat, there is still insufficient or inexistent commitment to properly understanding the meaning of illicit trade and how it operates in reality, and scarce academic efforts to address illicit trade through international and WTO law. This dissertation aims to reduce this gap by presenting a comprehensive study on defining and combating illegal trade from a rules-based perspective. In this effort, this thesis adopts a theoretical and practical approach to address these research concerns. Its first chapters frame the problem of illicit trade and propose a definition of illicit trade and a set of typologies designed to understand better how it operates. Then, from a real-life perspective, it discusses the complexities of cross-border illicit trade operations and the measures adopted by states to combat them. The research then addresses Illicit trade under international law to show that there is an international law regime specially devised to fight against illicit trade: the International Law Against Illicit Trade (ILAIT). Afterward, the dissertation revisits WTO law regarding illicit trade and the current standing of the WTO DSB on the illicit trade issue. Lastly, the dissertation analyzes, in line with ILAIT and WTO law, potential alternatives to deal with the illicit trade issue. The dissertation concludes that, even though WTO factfinders and adjudicators have not addressed illicit trade properly, the interpretation and application of WTO law, in accordance with ILAIT, provide sufficient tools to coordinate a more effective global response against illegal trade, also offering sovereign states enough regulatory room to combat international and domestic illicit trade.