After the death of his teacher Bartolo da Sassoferrato, in 1357, Baldo degli Ubaldi da Perugia emerged as the foremost Italian jurist of his generation. He was a leading proponent of the medieval ius commune, a transnational body of civil and canon law principles, methods, and jurisprudence. More than any other jurist of his day, Baldo adopted philosophical terminology and concepts from Aristotle and Thomas Aquinas to advance singular approaches and solutions to a wide array of new as well as old questions. The body of works he produced between the 1350s and 1400, ranging from omnibus commentaries on the Roman emperor Justinian’s Corpus Iuris Civilis, Pope Gregory IX’s Decretals, and feudal law, to individual tracts focused on particular topics (e.g., statutes, pacts, and notaries), and to several thousand legal opinions dealing with actual disputes (e.g., over papal elections, usury, citizenship, inheritance, illegitimacy, and dowries) left an indelible imprint on European jurisprudence.