Content-based ESL is a method that integrated English-as-a-second-language instruction with subject-matter instruction. The technique not only focuses on learning a second language, but on using that language as a medium to learn mathematics, science, social studies, and other academic subjects. Although this approach has been used for many years in adult, professional, and university education programs for foreign students, content-based ESL programs at the elementary and secondary school levels are just emerging. One of the reasons for the increasing interest among educators in developing content-based language instruction is the theory that language acquisition is based on input that is meaningful and understandable to the learner. Content-based ESL programs have been developed to provide students with an opportunity to develop their cognitive academic language proficiency, as well as to provide a less abrupt transition from the ESL classroom to an all-English-medium academic program. Content-based ESL courses provide instruction in the special language of the subject matter, while focusing attention as much or more on the subject matter itself. Examples of content-based instruction include: (1) mathematics, where communicating mathematical processes, concepts, and applications in English is emphasized; (2) science, where students learn to think, observe, classify, compare, communicate, measure, infer, predict, and identify space and time relationships in English; and (3) social studies, where students develop critical concepts in order to better understand the history and culture of the United States, as well as develop their cognitive skills. (TR)