Relative to the study of free trade agreements, customs unions (CUs) have been neglected in international law scholarship, despite the fact that by no means do they constitute a recent phenomenon. The present article aims to fill this gap by conducting a scoping analysis of the concept of customs union and identifying key issues in CU designs. The article problematizes what is understood by the concept of CU and what is entailed by the foremost definition of CUs, found in Article XXIV of the General Agreement on Tariffs and Trade (GATT). It further investigates how recurrent design issues are resolved in practice by different CUs considering the inherent tension between the enactment of common rules and institutions and state sovereignty. We find variety in the historical, economic and legal conceptualizations of CUs, flexibility and lacunas in Article XXIV GATT, and diversity of CU designs along with a discernible concern for the legal arrangements’ impact on state sovereignty.