My dissertation argues that musical theater and public debates about music shaped modern opinion in Spain during the last two decades of the eighteenth century, before the private and the public became institutionalized in the bourgeois family and the nation-state respectively. Musical debates were particularly stimulated by two converging circumstances in the last years of Charles III’s reign and the first years of Charles IV’s: the reopening of the Italian opera theater in Madrid, and unparalleled developments in the local periodical press. In examining short music theater pieces, printed criticism, and the conversation between the two media, I propose that during this time period the Madrid upper- and middle-classes first acknowledged opinion as a force capable of determining the functioning of their society. Like all intellectual endeavors during the late-eighteenth century, musical practices faced the challenge of reconciling the Habsburg legacy of a Catholic, monarchical Spain with the European Enlightenment that exalted human agency, reason, and nature. Unlike what happened in other European capitals, music in late eighteenth-century Madrid neither represented nor questioned the political establishment directly. Instead, public music performances at the city theaters provided society with a medium to digest and reformulate orally circulated opinions, Enlightenment-derived ideas, and Bourbon absolutist policies. In its capacity as a public language with multiple avenues for participation, music helped Spanish people to transition from the old to the new order without threatening their religious identity. In addition to contributing to the knowledge of Spanish music and criticism, this dissertation proposes that the late Spanish Enlightenment can be better understood through public cultural debates than through the official history of Bourbon reforms and enlightened despotism.