In this dissertation, I examine the historical development of the conceptual foundations upon which the contemporary field of international criminal law and the politico-legal project of international criminal justice have been built. While the legal doctrines and institutional practices of contemporary international criminal law did not decisively emerge until the early- to mid-20th century, in this dissertation I show that the ideas behind this institutional shift were already well-worn by then. Employing the tools of intellectual history and legal sociology, I explore the ways in which legal actors (including lawyers but also state officials, diplomats, military officers, merchants, planters, sailors, and others) framed, enacted, and deployed three foundational legal ideas over the course of the 19th century. In Chapter 2, I examine the ways in which the concept of a supranational or universal crime (the idea of supranational or universal crimes (acts whose commission merits both criminal accountability and the concern of publics and actors beyond the confines of any nation state) was used in public and political discourse over the course of the 19th century. Through a corpus analysis of over 6500 newspapers, pamphlets, and other quotidian texts containing phrases referencing one or another form of supranational or universal crime (e.g. “crime against humanity” or “international crime”), I show that these phrases were in common usage throughout the century and that they were generally used in ways largely similar to the way they are today, referring to “mass atrocities” and crimes whose commission or effects were transnational. In Chapter 3, I examine the ways in which legal actors experimented with the idea of international criminalization (the practices by which legal actors could define and establish certain forms of violence as “crimes of international concern”) before and during the 19th century. In this, I focus in particular on the emergence and proliferation of suppression treaties, a key legal tool with which international actors experimented in creating and codifying new international crimes through bilateral or multilateral agreements, first showing that examples of this genre of treaties can be found as early as the 1640s – more than a century before the earliest example discussed in the existing literature – and then discuss how British advocates and policymakers adopted and adapted this previously obscure legal tool starting in the first decades of the 19th century as a means to internationalize the abolition of slavery and the slave trade. Finally, in Chapter 4, I examine the ways in which legal actors experimented with the idea of internationalized criminal adjudication (the practices by which states might go about holding individuals accountable for such “crimes of international concern”) during the 19th century. To this end, I present four case studies in which political, military, and legal actors responded to alleged crimes of international concern by creating ad hoc internationalized courts and commissions of inquiry charged with adjudicating the criminal responsibility of alleged perpetrators. Engaging in this kind of foundational study of the conceptual underpinnings of international criminal law and international criminal justice is worthwhile because popular understandings of international criminality during the 19th century, and efforts by legal actors to implement and apply this concept during this period, not only shaped the boundaries between legitimate and illegitimate violence in the Western social imaginary but also prefigured and constrained the ways in which these concepts are later taken up by professional legal actors and formalized as part of the contemporary international legal order.