This chapter deals with the potential contribution of recent linguistic and psycholinguistic research to our understanding of the ways in which bilinguals (and multilinguals) process language data. The term “processing” takes on two meanings here, a developmental meaning and an on-line use meaning. Firstly, in the building up of knowledge representations (development), learners' acquisitional mechanisms need to process relevant linguistic information coming in from the environment. This is developmental processing. However, once knowledge representations become available to learners as users of their knowledge then those knowledge representations in turn need to be processed for the various uses to which learner/users want to put their language system at given moments in time. This on-line control may be called knowledge processing.

In considering various aspects of developmental processing and knowledge processing in bilinguals, extrapolations will be made from some current views on how language is represented in the mind of the learner/user and reference will be made to theoretical notions from sister disciplines which have not, at least not until very recently, been developed and demonstrated in a second language context. The theoretical status of various different kinds of linguistic knowledge will be considered, ranging from the more implicit knowledge inherent in the Chomskyan notion of linguistic competence through pragmatic knowledge, to characteristically explicit knowledge about language that is freely accessible to introspection, namely meta-knowledge.