Modeling of linguistic knowledge generally involves the compartmentalization of grammar into phonological, morphological, lexical, syntactic, semantic, and pragmatic components. These components are not isolated but interacting components. It is the resulting interfaces between grammatical components that forms the main topic of this volume, discussed from the perspective of bilingual L1 acquisition in early childhood and L2 in adulthood, as well as L1/L2 in late childhood. The book contains ten contributions by members of the Research Center on Multilingualism at the University of Hamburg and by other international scholars, all of them experts on multilingualism. Several pairs of languages are dealt with, among them Spanish and German, Mandarin and English, French and German, Italian and German, Turkish and English, Turkish and German, Dutch and Turkish, as well as Spoken German and German Sign language. Throughout the volume the central issue is that of representation at the interface of grammatical components.