After a review of the problem of predicting foreign language success, this booklet describes the development, refinement, and validation of a battery of psychological tests, some involving tape-recorded auditory stimuli, for predicting rate of progress in learning a foreign language. Although the battery was developed for more general application in high schools and colleges, this article focuses on the results obtained in a variety of "intensive" or semi-intensive foreign language courses in a variety of Western and non-Western languages, mainly under governmental auspices, as in the Air Force or at the Foreign Service Institute. Validity coefficients for the tests often reached high levels, with multiple correlations as high as .84. Results suggest there are at least four main components of foreign language aptitude: phonetic coding ability, grammatical sensitivity, rote memory for foreign language materials, and inductive language learning ability. Aptitude is general over different languages, and the tests offer diagnostic possibilities. Results are interpreted in terms of a model that depicts relations among aptitude, ability to understand instruction, motivation, time allowed for learning, and quality of instruction. An appendix of test descriptions and a list of references accompany the text. (Author)