Northwest Caucasian languages
His main works are devoted to comparative historical grammar of Slavic, Baltic, Iranian, Indo-European and Nostratic languages, comparative historical accentology and historical typology of accent systems, accentology of Northwest Caucasian languages and Central Saharan languages, and Japanese. He made a significant contribution (along with Vladislav Illich-Svitych, Sergei Nikolaev, Andrey Zaliznyak) to the reconstruction of the ancient Balto-Slavic accent system.