The papers in this collection present a view of language as an evolving system within which flexibility is systemic and subject to progressive change, driven by pressures of every-day coordinative action as in conversational dialogue. As these papers demonstrate, these pressures provide a force for variation, hence for adaptation, and change. Topics covered include formal and game-theoretical models of evolution, diachronic studies of change (semantic, syntactic, lexical) and the role of dialogue factors in promoting such change, psycholinguistic experimental methods probing the nature of coordination in both language and non-language based interaction, formal modelling of dialogue, and probabilistic studies of language variation. Authors include highly influential researchers: Joan Bresnan, Devyani Sharma and Ashwini Deo; Robin Cooper and Aarne Ranta; Ronnie Cann and Ruth Kempson; Miriam Bouzouita; Jonathan Ginzburg and Zoran Macura; Patrick Healey; Gerhard Jaeger; Simon Kirby, Hannah Cornish and Kenny Smith; Staffan Larson; Elizabeth Traugott. Together these papers present a cross-disciplinary synergy reflecting the view that human linguistic behaviour is essentially coordinative, and, within this general perspective, that language is a system of intrinsic flexibility, hence perpetually in a state of flux.This view involves a radical shift in assumptions away from the traditional view that language competence is a static system relative to which the dynamics of language performance has to be defined; and each paper, in its own way, explores what is involved in modifying such a clear-cut division.