The term Aktionsart is widely used in modern aspectology; however, there is no common understanding of its meaning. In basic terms, three different approaches to Aktionsart can be identified in modern literature on aspect. The most widespread view equates Aktionsart to lexical aspect, a classification of verbs into Vendlerian or quasi-Vendlerian categories that underlie aspectual properties of the verb. While this approach is predominantly used outside Slavic linguistics, Slavists usually apply the term to phenomena other than lexical aspect. In Slavic studies, the two existing approaches trace their roots to Sigurd Agrell’s study of Polish verb derivatives with special attention to the semantic functions of prefixes (Agrell 1908). However, Agrell’s definition of Aktionsart did not enable a unified understanding of the term. On the one hand, he described Aktionsarten as semantic functions specifying the way the action is performed; on the other hand, his analysis focused on derivatives rather than simple verbs. This triggered the development of the aspectual-semantic and the derivational-modificational theories of Aktionsart. The article discusses both approaches and the relationship between Aktionsarten and the aspectuality domain.