From 11 to 13 September 2017, the members of the Polyslav Association of Slavists gathered for the 21st annual International Academic Conference of the Polyslav Association. The conference was held at the Faculty of Philosophy of the University of Novi Sad, Serbia - the association's first meeting in a South Slavic country. Polyslav-XXI was attended by over 60 participants from research centres established in eight countries - most of them from Russia, Poland, and the conference organizing country, Serbia, but also from Slovenia, the Czech Republic, Hungary, Bulgaria, and Japan. The present proceedings contain 33 papers in eight Slavic languages (Russian, Polish, Serbian, Slovak, Ukrainian, Slovene, Bulgarian, and Ruthenian), and four in English. All 37 contributions - devoted to a vast variety of topics, including, for instance, the Slovak language spoken in Vojvodina, the Northeastern Bulgarian dialect in Romania, and the forms of the name of Muhammad in medieval Russian sources - meet basic publishing standards and international editorial conventions, including a double-blind review. A total of 51 reviewers - 44 from Europe and 7 from the USA - guarantee a high scientific level for the published papers.