The term multicompetence describes “the compound state of a mind with two grammars” (Cook, 1991a, p. 112). This paper reviews evidence addressing two questions:

1. Do people who know two languages differ from people who know only one in other respects than simply knowledge of an L2? L2 users differ from monolinguals in L1 knowledge; advanced L2 users differ from monolinguals in L2 knowledge; L2 usershave a different metalinguistic awareness from monolinguals; L2 users have different cognitive processes. These subtle differences consistently suggest that people with multicompetence are not simply equivalent to two monolinguals but are a unique combination.

2. Do people who know two languages have a merged language system rather than two separate systems? The L1 and L2 share the same mentallexicon; L2 users codeswitch readily; L2 processing cannot be cut off from LI; both languages are stored in the same areas of the brain; L2 proficiency relates to L1 proficiency. This evidence suggests merged systems at some level in some areas, even if some of it is open to other interpretations.

A final section discusses more general issues. Much SLA research is biased by adopting the monolingual as a norm rather than the multicompetent speaker. Multicompetence distinguishes diachronic transfer during the learner's acquisition from synchronic transfer between the two languages at a single moment of time. Multicompetence starts when there is systematic knowledge of an L2 that is not assimilated to the L1. Holistic multicompetence is seen as an offshoot of polylectal grammar theory applied to monolinguals. Language teaching should try to produce multicompetent individuals not ersatz native speakers.