In this course text on second language acquisition, the latest research of Halle and Chomsky, Lenneberg, Hatch, Larsen-Freeman, Dulay and Burt, and Krashen is presented. The text covers such topics as the effects of environment, age, and personality on second language acquisition; the role of the first language; and error analysis. Enough has been learned from the research to suggest that the learner's contribution to the whole learning process has been highly underestimated, and that a knowledge of these contributions can help improve current teaching practices. Theory is related to practice by the presentation of information on curriculum planning and classroom procedures. The chapters include: The Language Environment, Internal Processing, Effects of Personality and Age on Second Language Acquisition, The Role of the First Language, Transitional Constructions, Errors, Acquisition Order, Special Constructions, Aspects of Second Language Research Methodology, and From Research to Reality: Implications for the Teacher. Study questions and suggestions for further research follow each chapter. It is intended that the information about natural learning processes will help teachers devise effective lessons and develop materials that enhance natural development tendencies. (NCR)