The judgment delivered by Trial Chamber I of the International Criminal Court on 14 March 2012 in the case of The Prosecutor v. Thomas Lubanga Dyilo was welcomed by a number of experts as a landmark decision. Not only was the judgment the first ever adopted by the Court, thus marking a new step in the operationalization of the Rome Statute, but it also provided an opportunity for addressing a number of procedural and substantive issues that are essential to the progressive development of both international criminal and humanitarian law. This chapter does not seek to provide a comprehensive analysis of all these issues. It focuses instead on the core of the judgment, i.e., the definitions of the war crimes for which Thomas Lubanga Dyilo was convicted (Sect. 4.3). The Trial Chamber found that the accused was guilty of conscripting and enlisting children under 15 into an armed group, namely the Forces Patriotiques Pour la Libération du Congo (UPC/FPLC), and of using them to participate actively in hostilities. These crimes occurred in the Ituri region of the Democratic Republic of the Congo from September 2002 to August 2003. This article also examines how the Trial Chamber addressed the preliminary question of the characterization of the situation during the relevant period (Sect. 4.2). The judgment provides essential insight into the Trial Chamber's understanding of the notion of armed conflict within the framework of the Rome Statute.