A three-year research project investigated the effects of instructional practices and patterns of language use in bilingual and English-only classrooms on general academic development and the development of English language skills by limited-English-proficient students, especially those skills needed to participate fully in the society's schools. The subjects were native Chinese- and Spanish-speaking students with two to three years of exposure to English, from 17 bilingual program classes. Tests, classroom observation, and audio and video recordings focusing on instructional situations were used to assess oral and written language comprehension and production, subject-area academic achievement, and the instructional practices and patterns of instructional language use. It was found that variables influencing development of English production skills included: interactional opportunities, especially for Hispanic students and those with lower initial English proficiency; quality of the learning environment, especially for Chinese students with low initial English proficiency; and quality of language and teaching, largely for Hispanic students. Variables affecting English comprehension skills included: percent of time in teacher-directed activity; quality of learning environment, for Hispanics at all levels and for Chinese at the intermediate level; quality of instructional language, only for Hispanics; interactional opportunities, for Hispanics; and verbal and extended-response practice, for the Chinese. (MSE)