1. Introduction; 2. Becoming Bilingual in Infancy: Focus on Bilingual First Language Acquisition; 3. Bilingualism in Early Childhood: Bilingual First and Early Second Language Acquisition; 4. Bilingualism in Middle Childhood: BFLA, ESLA, and SLA; 5. Socioeconomic Status and Bilingual Development in Childhood; 6. Summary and Conclusion.