In the field of second language (L2) acquisition, researchers as well as L2 teachers have long been aware of the difference in lexical usage patterns and in the meaning ascribed to L2 words by native speakers and by L2 learners. It has been observed that L2 learners appear to use more words of general than of specific meaning, tend to both overextend and underextend word meanings, and often fail to respond to constraints of register and collocation. In the case of polysemous words, it is also found that L2 learners acquire some meanings but not others. Idioms and figurative language present great problems for L2 learners. All these facts seem to indicate there are constraints on L2 learners' lexical acquisition. However, what these constraints are is not fully known.