The author combines a syntax-theoretical treatment of telicity marking and an empirical study of the second language acquisition of English telicity marking by native speakers of Bulgarian, a Slavic language. It is argued that Vendler’s lexical classes of verbs (states, activities, accomplishments and achievements) can be represented in four phrase structure templates, where lexical properties of the verb and of the object compositionally determine telicity. A parameterized distinction between English and Slavic aspect is proposed. The book addresses two major acquisition issues: (1) what is the nature of the initial hypothesis Bulgarian learners of English entertain regarding telicity marking (i.e., is there native language transfer)? (2) are adult learners capable of resetting the telicity marking parameter? Both L1 transfer and parameter resetting are experimentally supported. In addition, the study investigates the L2 acquisition of a cluster of complex predicate constructions, purportedly related to the telicity parameter in the grammatical competence and in child language acquisition of English.