Teachers who participate in learning and teaching abroad enhance their intercultural competence, develop more globally informed and critical perspectives on education, and improve their foreign language and teaching skills. However, most of the research on teacher study abroad has been conducted with pre-service rather than in-service teachers. To address study abroad designed for working teachers, this study analyzes a month-long, instructor-led program in San José, Costa Rica in which 10 primary and secondary in-service teachers of English as a Second Language (TESOL) from New York City volunteered as English teaching assistants. Participants also engaged in Spanish language learning both formally through university coursework as well as informally through homestay. A conceptual framework for the design of such programs is suggested, drawing on the literature on teacher study abroad and transformative learning theory, with implications for the personal and professional learning of in-service teachers.