This paper proposes a constructivist account of the development of morphosyntax in bilinguals, based on an examination of two populations--English-Spanish bilinguals in Miami and English-Welsh bilinguals in North Wales. Despite sociocultural and sociolinguistic differences across these groups, the development of bilinguals in the two groups reveals core commonalities. These core commonalities are posited to be a by-product of normal processes of language development, as seen from a constructivist view. These processes are outlined, and predictions based on them are discussed regarding key issues in bilingualism research, including whether bilinguals' two languages develop as autonomous or interactive systems, whether bilingual development is parallel in pacing and sequence to monolingual development, and whether bilingual attainment in their two languages is similar to that of monolinguals.