This collection of articles provides theoretical foundations and perspectives for language attrition research. Its purpose is to enable investigations of L1 attrition to avail themselves more fully and more fundamentally of the theoretical frameworks that have been formulated with respect to SLA and bilingualism. In the thirteen papers collected here, experts in particular disciplines of bilingualism, such as neurolinguistics, formal linguistics, contact linguistics and language and identity, provide an in-depth perspective on L1 attrition which will make the translation of theory to hypothesis easier for future research.