The United States government is perennially in need of employees with proficiency in critical foreign languages to communicate with foreign counterparts and maintain relationships worldwide. In order to fulfill this need, the government devotes significant resources training federal employees to advanced levels of language proficiency through intensive courses aimed at developing communicative language skills that reflect the work that employees will perform in their work advancing the interests of the United States abroad. Notable proportions of employees fail to meet proficiency goals at the end of training, and little is known about what learner individual differences drive whether or not employees will meet their proficiency goals in order to perform their work on behalf of the United States. To this aim, the current investigation utilizes multiple analyses to explore and explain the interrelationships between learner individual differences, language difficulty, and proficiency attainment throughout training. The investigation constitutes two related analyses. First, a path-analytic approach examines associations between a cognitive (aptitude) measure and noncognitive (motivation, familiarity with curricula, previous advanced second language learning) measures with student proficiency achievement throughout training. A second analysis builds on the first: the path-analytic model incorporates a measure of difficulty of the language studied by the students to determine how difficulty influences language learning and ultimate attainment within the context of individual differences in L2 speaking and reading. Results demonstrated consistent influence of language aptitude on proficiency attainment, and notable influences of previous L2 acquisition and the alignment of training to individuals' language use goals. L2 difficulty moderated the relationships between individual differences and proficiency assessment scores during several points in training. The findings support an understanding of adult L2 acquisition that more fully considers learners' goals and previous L2 experiences and consideration of the impact that difficulty can have on individual learners' abilities to achieve target proficiency goals. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]