This booklet describes key principles of and research on teaching additional languages. The 10 chapters focus on the following: (1) "Comprehensible Input" (learners need exposure to meaningful, understandable language); (2) "Language Opportunities" (classroom activities should let students use natural and meaningful language with their classmates); (3) "Language Practice" (classroom activities should encourage students to use the additional languages for genuine communication); (4) "Learning Strategies" (students should be taught strategies that enable them to increasingly learn language on their own and from the classmates and others without their teacher's help); (5) "Listening" (students need practice in understanding naturally spoken additional languages); (6) "Speaking" (students need practice speaking in language comprehensible to others); (7) "Reading" (students need practice in comprehending natural texts); (8) "Writing" (students need practice creating effective, natural language that communicates their intended message); (9) "Grammar" (formal grammar instruction may be beneficial in some situations but not in others); and (10) "Comprehensible Pronunciation" (pronunciation instruction should make students understandable to other users of an additional language). Each section presents research findings, strategies for use in the classroom, and references. (Contains 33 references.) (SM)