A key notion in sociolinguistics, that of “repertoire” is to date the only one that fully accounts for the complexity of forms of multilingualism in the new migration contexts. After providing an overview of the existing literature in this field of research and a sketch of the linguistic resources used by new migrants, the heuristic relevance of the repertoire notion to describe second language structures and acquisition is dis- cussed. This will let the epistemological conflict between sociolinguistic and acquisi- tional approaches to multilingualism be overcome in future.