This paper reviews the literature on similarities and differences between first and second language lexical acquisition. After a brief discussion of differences in input, we go on to early lexical development, considering both the speed of acquisition as well as possible reasons for more efficient lexical learning in first language acquisition as compared to second language acquisition. We discuss the role of phonological representations in facilitating the extraction of units from incoming speech. We continue with a discussion of unanalysed units, arguing that their role as a stepping stone into language is much the same in first and second language acquisition. Finally, we review methods for investigating the first and second language lexicons.