This volume presents the results of a major investigation of second language proficiency in various groups of school age learners. The contributors, who are experts in the field of bilingual education and second language acquisition, address such issues as the nature of language proficiency, the impact of classroom practices on second language learning, the relationship between social-environmental factors and bilingual proficiency, and the relationship between age and language proficiency. This original collection of articles will be of interest to all those concerned with policy and practice in second language education.