Most adult second language learners believe that the most difficult aspect of learning a second language involves the acquisition of a vocabulary sufficient to meet their receptive and productive communicative needs. They recognize that the grammar and the sound system need to be incorporated, of course, but feel that it is the acquisition of vocabulary items that demands the most attention and requires the most time. Intuitively, they are probably right, although perhaps not for the right reasons. A typical learner (especially a classroom learner), for example, will state that what needs to be done in the acquisition of words is to memorize a form (either phonological or graphic) and its associated meaning. But the problem is much more complex than this. Learning a lexical item involves a number of different properties: the sound sequence and the meaning(s) associated with it, to be sure, but also the syntactic category it belongs to, its co-occurrence restrictions, and, if it is a verb, the number of arguments it can take, the thematic roles of the arguments, and how these arguments and their thematic roles may be encoded syntactically.

One would think, therefore, that language researchers would have already put in much time and thought on the question of how the lexicon is learned. But such is not the case, either in the study of first language acquisition or in the study of second language acquisition. And perhaps this is not surprising.