The main objective of this paper is to compare the process of vocabulary acquisition of monolingual children with that of simultaneous and successive bilingual children, to present in detail what they have in common and how they differ, and to further discuss phenomena which are specific to bilingual vocabulary acquisition. Its central conclusions are that vocabulary acquisition in monolinguals and simultaneous bilinguals is highly similar, whereas successive bilinguals show a delayed development. Furthermore, it is determined that to rate a bilinguals lexical competence only in one of their languages ignores the circumstances of the bilingual language acquisition.