This book provides a complex insight into how law, as a distinct tool and technology, conceptualizes and operationalizes race, ethnicity and nationality. The focus of the comparative project, by bringing examples from five continents and scores of jurisdictions, as well as showcases for hybrid, intersectional groups, is specifically the morphology and dynamics of legal categorization. Separate discussions concentrate on conceptualizing groupness and membership, as well as agency and contestation. The book shows that although identity politics has dominated recent decades, ethno-racial self-identification is not the only operationalizing model legal regimes apply, especially with the recent boost in artificial intelligence (AI), and bio-genetic research. Examples for the “re-biologization” of ethno-racial conceptualization are brought from a wide range of legal regimes, including citizenship, anti-discrimination, asylum, and Indigenous law. The work provides a journey through the administrative-political construction and contestation of ethno-racial classifications, with particular attention paid to the concepts of free choice of identity, covering, and fraud, as well as the arbitrariness, the historical path dependence, and the role of merit in conceptualization. While the starting point of the book is to capture ethnicity as a category of law, it shows how legal conceptualization and operationalization are intertwined with categories of analysis and experience. The methodology applied is comparative constitutional and international law, but the research will have wider interdisciplinary appeal offering a novel perspective for a broad audience in social sciences and humanities.