ABSTRACT
Second language acquisition (SLA), like applied linguistics, is an interdisciplinary field with widely varying interpretations placed on it, and varying demands made of it. SLA theory and research findings are utilized in many fields, including first language acquisition, theoretical linguistics, neurolinguistics, language learning in abnormal populations, language teaching, education, and psychology. The theories differ substantively, and in at least five other ways: source, scope (or domain), content, type, and form. The way in which SLA theories differ is by source, that is, in their origins inside and/or outside the field. Theories differ with respect to content and at a broader level, the relative importance accorded internal or environmental factors. The acquisition type and context of interest also varies. The embryonic language teaching theory of which focus on form is but a part is already more powerful than, S. D. Krashen's theory, because it has recourse to a mechanism, focus on form, to induce acquisition that Monitor Theory does without.
