The acquisition of a second language, be it by everyday communication or by instruction, follows certain principles, which stem from various properties of human language processing, from the learner's specific motivation and, finally, from the way in which samples of, or information about, the language to be learned are made accessible to the learner. The objective of second language acquisition studies is to uncover these principles.

Above and beyond the genuine interest which any human activity as common as learning and using a second language excites, there are two reasons why the study of second language acquisition is an important and sometimes rewarding enterprise. First, for foreign language teaching to be maximally effective, it must be tuned to the principles outlined above. To the extent to which we do not explicitly know them, successful language teaching can only be a matter of practical experience, of individual pedagogical gifts, or of luck. Second, the study of how the human mind builds up fragmentary linguistic systems from limited input, how it re-organizes them if new input becomes available, and how it uses them for communicative purposes for which they may still be inadequate, can tell us something about how human language processing functions in general.