The ways in which children learn a language--be it their mother tongue or their second language--can have a strong influence on their success in school. Researchers in linguistics and early child development have tried to determine the factors that can help and hinder language acquisition in young children, with some conflicting results. In this article, the author reviews the research and existing theories on language development, focusing on how pre-schoolers' social environment affects their ability to learn languages and their subsequent reading and writing skills. Because children from immigrant families and those with a low socio-economic status often have difficulty with language acquisition, this review looks in particular at language development initiatives for disadvantaged groups in daycare settings. Beginning with an overview of theories on language development, the author examines the sometimes-conflicting hypotheses that attempt to explain how children acquire their first language and how some become bilingual. Next, the role of the social environment is reviewed, beginning with the importance of verbal input for first language development, then looking at the complex phenomenon of second language acquisition. The author provides an overview of research on bilingualism and its various forms, focussing on children who learn a second language after they are already established in their mother tongue, as opposed to the less common cases of children who acquire two languages from birth. Looking in particular at research on migrant children, the paper explores the course and duration of second language acquisition, as well as the common linguistic behaviours that may arise. Conditions that influence children's adoption of a second language and culture are then examined, as well as similarities and differences between first and second language acquisition. Factors that influence the development of a second language such as age, motivation, interaction, educational style, socio-economic status, and experiences in daycare are discussed, and the impact of early language development on school success is examined. Finally, the author reviews several programmes aimed at fostering language development and literacy in infancy and early childhood in the United Kingdom, the United States, and Germany, and concludes with a discussion of the implications of the research, providing several recommendations for practice. A glossary is included. (Contains 24 footnotes.)