In this book we have critically but constructively reviewed the state of the art regarding languages in contact (bilingualism), from individual bilingualism (or bilinguality) to societal bilingualism. We began by examining traditional and current definitions of bilingualism, none of which was found to be adequate. They all show one or more of three main weaknesses. First, they are unidimensional: they describe the bilingual in terms of one dimension, such as language competence, ignoring other equally important aspects. Second, they fail to take into account the different levels of analysis, from individual to societal. Finally, they are not based on a general theory of language behaviour.

To remedy these failings we proposed a multidimensional theoretical model of language behaviour (Figure 1.1), which we also apply to bilingual behaviour and which guides us throughout the book. According to this model language processing operates at different levels of organisation which are embedded in one another, from micro- to macro-levels: these are the individual networks, the interpersonal networks, the social networks and the social structures. These levels are not independent of one another but are in dynamic interaction. Within and between these levels there are complex mappings of the forms of language behaviour onto the functions they are supposed to serve. It should be stressed that the social and the psychological dimensions are found at every level simultaneously, in the sense that any speaker is at one and the same time an individual, a member of social networks and groups, and part of the whole society. These different levels of language processing require different types of analysis at the individual, interpersonal and societal levels.