The overwhelming bias for investigations of bilingualism is to focus on the increase of knowledge and crosslinguistic traffic from the L1 to the L2. Developments which concern loss, deterioration or reduced accessibility of knowledge and traffic from the L2 to the L1 are much less well-studied and understood, and are usually treated as a somewhat marginal issue. The present contribution provides an overview of research in first language attrition and argues that changes to the first language system are part and parcel of the development of bilingual knowledge and processing. As such, they can help provide additional insight into controversial issues, such as questions about the existence of maturational constraints in L2 learning, and potentially help resolve these matters.