This monograph presents language factors related to bilingualism, to assist educators to distinguish normal phenomena due to bilingualism from symptoms of a handicapping condition. The first section, called "The Nature of Bilingualism," discusses the diversity of bilingual students and their dynamic use of two languages, focusing on simultaneous and sequential bilingual children, first language acquisition in bilingual children, language varieties, language loss, codeswitching, and cognitive development in bilingual children. The section entitled "Second Language Development" explores the role of primary language interference; the similarity between the processes of first and second language acquisition; the rate of second language acquisition; and the factors of age, access to peers, cultural group membership, personality and individual strategies, motivation and attitude, and formal instruction in English as a second language. The final section, "Language Proficiency," examines relative language proficiency and language dominance, presents a model of bilingual proficiency, and reviews the problems of comparably limited bilingual children. A list of 64 references is appended. (JDD)