The Social Lives of Adolescent Study Abroad Learners and Their L2 Development LUZIA SAUER 1 and ROD ELLIS 2 1 Zurich University of Teacher Education, Department of Foreign Language Education, Lagerstrasse 2, 8090 Zürich, Switzerland Email: luzia.sauer@phzh.ch 2 Curtin University, School of Education, Kent Street, Bentley, Perth, Western Australia, 6102 Email: Rod.Ellis@curtin.edu.au This article reports a longitudinal case study of two German high-school exchange students’ 5.5-month study abroad (SA) in New Zealand, examining their social lives and oral second language (L2) devel- opment. Six informal interviews, weekly diary entries, and monthly summaries were used to elicit data about their overseas experiences and refections associated with them. The qualitative analysis investi- gated the nature of the students’ social lives at school, in their homestay, in their hobbies, and during their free time. The results demonstrated that opportunities for L2 engagement varied considerably with context—some affording and others restricting interaction. Especially the presence of co-nationals impeded L2 interaction and required the students to seek out opportunities themselves—an effort that the students were not always willing to make. For the quantitative analysis, the interview data were coded using multiple measures of complexity, accuracy, lexis, and fuency (CALF). Compared to previous stud- ies, the results showed that all CALF dimensions improved during SA, but just for one student and not in a linear fashion. The two learners’ L2 profles varied considerably, demonstrating that development is dynamic and unique and that interpretations of it need to take account of the learners’ social contexts and what they make of these. Keywords: study abroad; adolescents; high school; second language development; CALF; social lives; social identity; social networks; communities of practice THE STUDY ABROAD (SA) CONTEXT IS considered particularly fruitful for investigat- ing second language (L2) acquisition given the potential for language learning opportunities (Churchill & DuFon, 2006). Many studies have thus attempted to document whether SA leads to gains in L2 profciency, often in comparison to stay-at-home students (e.g., Mora & Valls–Ferrer, 2012; Serrano & Tragant, 2016). Researchers have also documented the ways that learner-external as well as learner-internal factors infuence students’ language development during SA. Researchers in- terested in the former have typically examined the extent to which SA sojourners interact with The Modern Language Journal, 0, 0, (2019) DOI: 10.1111/modl.12589 0026-7902/19/1–24 $1.50/0 C  National Federation of Modern Language Teachers Associations locals and the social factors that promote and in- hibit this (e.g., Trentman, 2015). Those interested in the latter have tended to investigate individ- ual learner differences—particularly motivation, aptitude, and identity. Few studies, however, have adopted a case-study approach with a longitudinal focus on the relationship between social or indi- vidual factors and changes in L2 profciency (but see Isabelli–Garcia, 2006; Kinginger, 2008, 2009; Spenader, 2011). The study we report in this article docu- ments the changing nature of the social lives of two adolescent learners of English during their 5.5-month SA in New Zealand, pinpointing the so- cial identities that fgured in the efforts they made (or did not make) to join local communities of practice and the learning opportunities these af- forded. We will also document changes in their L2 English, plotting fuctuations in the complex- ity, accuracy, lexis, and fuency (CALF) of their