The aim of this paper is to introduce activists and scholars to efforts by the Global Detention Project to construct rigorous data on the facilities used to detain irregular migrants and asylum seekers in detention as they await deportation or to have their claims assessed. The paper proposes a set of conceptual tools that can be used to study the evolution of detention systems and undertake comparative analysis of national detention estates. Using a broad assortment of material—including legal opinions, international norms, on-the-ground reports, and academic and human rights literature—the paper proposes a carefully circumscribed definition of migration-related detention as well as a framework for developing data on various dimensions of this practice. It then discusses in detail a selection of variables to demonstrate the utility of an empirically-grounded approach to scholarship and advocacy on this issue.