This article is a review of the literature on the use of cultural-historical activity theory (CHAT) in the context of language instruction. It explores the concept of mediation as applied to the learning of a second language. First introduced by Vygotsky (1978) and expanded upon by researchers such as Engeström (2015) and Prenkert (2010), this concept is often presented by researchers in the field of second language teaching without being operationalized. Stemming from this observation, the objectives of the present article are to redefine the concept of mediation, to circumscribe its role in the activity system, and to describe its role in second language teaching research. It also aims to highlight the new applications of mediation as used in the second language classroom. To do so, a narrative literature review based on data bases (LLBA, ERIC, EBSCO, PsycNET, SOFIA) and inclusion criteria is carried out. The texts on which this literature review rests are empirical in nature and operationalize mediation in context. The analysis of the texts reveals a shared definition of mediation and a common idea as to the role of mediation in the second language classroom. In this regard, it draws parallels between the different elements of the activity system. Finally, several contributions of the selected texts are revealed: (1) the dual role of language as mediator and object, (2) the role of learning strategies as mediators, (3) the role of class culture as a mediator, and (4) the use of new artefacts in the classroom.