This special issue explores the relevance of Dynamic Systems Theory ( henceforth DST) to research in first language attrition (henceforth LI attrition). While research in LI attrition has traditionally drawn on paradigms in other areas of linguistic enquiry, e.g. approaches current in L2 acquisition studies, important developments across a range of disciplines have paved the way for a fundamen- tally different conceptualisation of language development. In this special issue, DST guides our enquiry into LI attrition, which we view as one specific dynamic process of change in language development.DST was first linked to LI attrition by Herdina and Jessner (2002), and then further developed by de Bot, who suggested that DST provides 'a fresh and promising perspective on language attri- tion as a normal part of language development and not as a lamentable loss of what once was' (2004, p. 233). De Bot argued that a DST-based model of language development should be appli- cable to language attrition and that the theoretical underpinning of LI attrition research would be strengthened by adopting this perspective (see also Ecke, 2004). However, de Bot also pointed to methodological challenges implicit in applying a DST perspective to LI attrition. Here, one of the most salient challenges is actually testing DST, a feat yet to be accomplished. Nevertheless, eight years on, the purpose of this special issue is to present for the first time empirical research which has adopted the DST framework to study LI attrition across the lifespan.In this introduction, two propositions related to the relevance of DST in LI attrition research are discussed.(1) LI attrition as such can be interpreted as evidence against the view that once an LI is acquired 'in full' (i.e. throughout childhood and up to the onset of puberty), it displays invariance which is not evidenced in a second language (henceforth L2, i.e. a language acquired post-puberty). Instead, it is proposed here that L1 attrition /as such/ is evidence for the view that the LI develops dynamically across the lifespan.(2) LI attrition itself is dynamic. As it unfolds, often in the context of a life-changing event such as migration to a new country and attendant L2 acquisition, it does so dynamically in a process of constant change and in interaction with the developing L2 and environment.We define and contextualise key concepts (LI attrition and DST) in order to relate them to our two propositions. We further discuss important features of DST in relation to the contributions of this special issue, as well as some methodological challenges relevant for studying the dynamics of LI attrition across the lifespan.LI attritionWe delimit LI attrition to non-pathological, non-age related changes in the LI of a late bilingual, which often entail reduced proficiency and loss of structural features previously acquired (see Kopke & Schmid, 2004; Schmid, 2011). While LI attrition may potentially occur for reasons not involving another language, e.g. the 'desert island situation' (Sharwood Smith & van Buren (1991, p. 22), it usually unfolds alongside the development of an L2, for example after relocation to a new country where the ambient language is acquired. Such relocation may involve either being entirely cut-off from the LI, as often happens with adopted children (Isurin, 2000), or at least a reduction in contact with the LI.Moreover, LI attrition contrasts with language change processes at the group or societal level, such as intergenerational language shift, although it may be a precursor to, and share certain simi- larities with, such macro processes. Accordingly, a conceptual and terminological distinction between micro-level LI attrition and macro-level language shift under the cover term of language loss has been drawn (de Bot, 1998).Lowered proficiency in the LI, or linguistic behaviour that differs from 'typical speakers' (Cook, 2003, p. …