This one-week course compares the linguistic development of young bilingual children (0-4 years) with that of monolinguals. We will discuss theoretical approaches to and empirical studies of language system differentiation in the lexicon, but in particular on the morpho-syntactic level. Is there evidence for an initial 'Single System' (e.g. Volterra & Taeschner 1978), or for 'Separate Systems' (e.g. Genesee 1989, Meisel 1989) in the children's productions? Once language-specific grammars emerge, are they consistently kept apart by the bilingual child, and what does this tell us about the linguistic capabilities of the human mind? We will discuss input and contextual factors, the significance of code-mixing, word order transfer and other types of cross-linguistic influence (cf. e.g. Bohnacker 1999, de Houwer 1990, Goodz 1989, McClure 1997, Meisel 1989, 1994, Paradis & Genesee 1996, 1997, Pearson et al 1995, Petitto et al 2001, Quay 1995, Tracy 1995, Yip & Matthews 2001).