How do children develop bilingual competence? Do bilingual children develop language in the same way as monolinguals? Set in the context of findings on language development, this book examines the acquisition of English and Spanish by two brothers in the first six years of their lives. Based on in-depth and meticulous analyses of naturalistic data, it explores how the systems of both languages affect each other as the children develop, and how different levels of exposure to each language influence the nature of acquisition. The author demonstrates that the children's grammars and lexicons follow a developmental path similar to that of monolinguals, but that cross-linguistic interactions affecting lexical, semantic and discourse-pragmatic aspects arise in Spanish when exposure to it diminishes around the age of four. The first of its kind, this original study is a must-read for students and researchers in bilingualism, child development, language acquisition and language contact.