The article appears in a handbook that demonstrates the interconnection between language and ethnic identity, providing a systematic treatment of language and ethnic identity efforts, assessing their relative successes and failures, and placing the cases on a success-failure continuum. This essay focuses on the early nineteenth-century Illyrian Movement attempt in the framework of Pan-Slavism—an ideology intended to unite spiritually all Slavic speakers—to unify the South Slavs (Croats, Serbs, Bosnians, Montenegrins, Slovenes, Macedonians, and Bulgarians) by creating a common literary language for them. The Zagreb-based Movement, which lasted from 1835 to 1848, was driven at its beginning largely by the activity of its charismatic leader, Ljudevit Gaj, in response to rising Hungarian nationalism and assimilatory practice that had threatened to erase Croatian identity. Acting on a widespread impulse among central European Slavic intellectuals, the Illyrian Movement offered a more extensive solution to the problem than the Croatian patriots and neighboring Slavic groups were ready to accept. Though the Illyrian Movement was abandoned by the 1848 Revolution, the impulse to merge South Slavic nations resurfaced in the late-nineteenth and early twentieth-century Yugoslav Movement, which had its ideological roots in the Illyrian Movement.The Illyrian literary language as such was abandoned, though in Croatia the principles of its construction have persisted and even reemerged with vigor in the post-Yugoslav period; it has also left traces elsewhere throughout the South Slavic standard languages. The Movement failed to integrate the Slovene lands, whose inhabitants consolidated their national identity around the language of Carniola; nor did it draw in Serbia and Montenegro, which followed a different vision of language standardization. On the other hand, the Illyrian Movement laid the foundation for the rapprochement of Croatian and Serbian, whose standard forms are based on a common dialect, and led also to the political construct Yugoslavism. Consequently, as a Croatian national program, the Illyrian Movement may rank 10 on the success scale; as a program to unite all the South Slavs, perhaps 5