The chapter provides an introduction to the sociocultural analysis of second language (L2) acquisition as a process of socialization by and through the learning of an additional language. It offers a discussion of socio-cultural institutions and communities involved in L2 learning as well as a description of the development of language repertoire in additional language learning as a social accomplishment. The chapter opens on a summary of the controversy that arose in the early 2000s between the cognitive and the socially-oriented Second Language Acquisition (SLA) paradigms. Bypassing this divisive debate, the chapter focusses on the diversity of approaches to additional language learning within the sociocultural paradigm. Key concepts of sociocultural SLA research such as social integration, agency, identity and power relations as well as affordance and language repertoire are defined. A framework of analysis for the learner’s language repertoire is presented. Finally, the chapter recalls issues in the study of naturalistic SLA as a privileged area of research in light of the sociocultural paradigm. Longitudinal data from one migrant worker are analysed to illustrate the social and linguistic development of his language repertoire. In conclusion, the chapter charts some pending issues for the sociocultural analysis of L2 socialization.