Research on second language acquisition and bilingual development strongly suggests that when a previously monolingual speaker becomes multilingual, the different languages do not exist in isolation: they are closely linked, dependent on each other, and there is constant interaction between these different knowledge systems. Theoretical frameworks of bilingual development acknowledge this insofar as they usually draw heavily on evidence of how the native language influences subsequent languages, and how and to what degree this influence can eventually be overcome. The fact that such crosslinguistic transfer is not a one-way street, and that the native language is similarly influenced by later learned languages, on the other hand, is often disregarded. We review the evidence on how later learned languages can re-shape the L1 in the immediate and the longer term and demonstrate how such phenomena may be used to inform, challenge and validate theoretical approaches of bilingual development.