This review examines the second language acquisition (SLA) literature with regard to the role of “massively multiplayer online games” (MMOGs) in second language (L2) learning. It focuses on commercially developed off-the-shelf (COTS) MMOGs only (some of them modified for educational purposes such as Reinders’ & Wattana’s work). It surveys the current empirical research to find out which aspects of L2 learning have been investigated, how they were studied, and what the findings suggest in relation to L2 learning opportunities and outcomes within and beyond MMOG contexts. We synthesized the findings of 31 studies reporting empirical evidence about the role of MMOGs in L2 learning. We observed that the empirical research in this area is mainly qualitative and that L2-related motivational and affective factors, L2 vocabulary, and learners’ communicative competence (or discourse management strategies) are the most widely investigated topics. Based on the findings, our paper presents a model that depicts hypothetical interrelationships among (a) MMOG designed settings, (b) the social and affective affordances provided in these settings, (c) L2 learning opportunities, and (d) the L2 learning outcomes achieved. We conclude that MMOGs provide socially supportive and emotionally safe (i.e. low-language-anxiety) environments that afford multiple opportunities for L2 learning and socialization, which, in turn, help L2 learners to enrich their L2 vocabulary repertoire and enhance their communicative competence in the target language.