Recent years have brought increasing attention to studies of language acquisition in a country where the language is spoken, as opposed to formal language study in classrooms. Research on language learners in immersion contexts is important, as the question of whether study abroad is valuable is still somewhat controversial among researchers (DeKeyser 2007; Sunderman & Kroll 2009). In the introduction to a pioneering volume on language study abroad, Freed (1995, pp. 17-18) noted that a vital question concerns the relative linguistic benefits of a summer, a semester or a year in the foreign environment. Our purpose in this colloquium, which was organized by Lynne Hansen, was to introduce a new line of research which allows comparisons of L2 attainment over these exposure times as well as longer periods. Tens of thousands of young missionaries for the Church of Jesus Christ of Latter-day Saints (also known as the LDS or Mormon Church), advanced speakers of some fifty languages, return home annually after two years of language learning abroad. This natural sample of learners, in its relative uniformity of learner characteristics and learning contexts, allows for the control of variables in SLA research which can be problematic in studies of more heterogeneous groups.