Opera counts among the very few genres in the history of artistic creation in which theory came before practice: rather than recognized as a genre after it had been in existence for some time, as usually happens, opera was famously “invented.” At the same time, perhaps no other genre relies on convention as heavily as opera. How could such a convention-bound genre be “invented”? This chapter attempts to answer this question, discussing first the “invention” of opera, and then the ways in which the category of genre has been employed in the aesthetic and historiographical discourse on opera, with special focus on a few emblematic cases such as Don Giovanni, Fidelio, Carmen, Wagner’s music dramas, and modernist music theater.