Three aspects of bilingual code-switching are examined: (1) code-switching as an integral part of bilingual behavior, especially in early stages of language acquisition; (2) social factors influencing the child's ability to differentiate languages and make an appropriate language choice; and (3) hierarchical organization of these social factors, based on their order of emergence and relative significance in affecting language choice. Data were obtained from ten-year longitudinal studies of two children raised bilingually in Spanish and English. The findings suggest that while the social factors in each case vary greatly, children learn early to discern the factors that are significant for their own context and guide the choice of language. Although bilinguals alternate and even mix codes, they also know in which instances to make separate linguistic choices regardless of their proficiency in the second language. Families are found to play a critical role in developing the patterns for early bilingual behavior and insuring its continuance while the mainstream language becomes more dominant in the child's life. Separate language use appears to aid language differentiation and bilingual development, while continuous language mixing may encourage passive bilingualism and lagging development in one or both languages. The evidence is found to favor maintaining language distinctiveness. (MSE)