The paper considers the results of theory and research in the psychological aspects of foreign language teaching as sources of guidance in making sound eucational decisions concerning the aims of language teaching at all levels. Major attention is given to the suggestion offered by research findings that language aptitude depends upon a somewhat different set of abilities (primarily differences in learning rate, from those articles required to master other subject matter. An explanation of the Modern Language Aptitude Test's ability to measure, predict, and identify such distinctive foreign language learning traits as (1) phonetic coding ability, (2) grammatical sensitivity, (3) rote learning ability for meanings of foreign language words and expressions, and (4) deductive learning ability, precedes a discussion of the Test's use for selection, guidance, and diagnosis. More briefly examined are how the factors of age, length of study, and motivation influence the learning of a foreign language. Concluding remarks concern the proper aims for language instruction and the teaching methods appropriate for achieving these goals. For related documents see FL 001 222 and FL 001 223. (AF)