The U.S. Military Services employ thousands of servicemen and women in language-related positions that are critical to the nation’s national security. These positions require personnel with high-level capability in various languages and dialects (Asch & Winkler, 2013). A complex accession and training system that begins at local recruiting stations across the nation leads to worldwide placement of language professionals who serve multiyear tours in the U.S. Air Force, Army, Navy and Marine Corps. High levels of cognitive ability, as measured by two cognitive aptitude batteries, one general (ASVAB) and one language (DLAB), are required for selection into these positions. Following significant investments in basic levels of training, the jobs themselves demand high level skills, and the service members find themselves constantly challenged to grow their skills. Traditional research on the effectiveness of the accession and training processes focuses on learning outcomes, rather than growth. This research used a longitudinal design to investigate how general aptitude, language aptitude, non-cognitive and language distance measures impact language proficiency growth. Hierarchical linear models and hierarchical generalized linear models were used and the significant findings were similar. The study found that overall, while language test scores followed a drop-and-recover pattern, there was very little growth overall. Three aptitude subtests, one from ASVAB (Mechanical Comprehension) and two DLAB subtests (Part 3 and Part 4) were found to constrain initial growth in the listening modality. Language distance was found to constrain initial and subsequent growth in listening and reading.