This edited volume is devoted to expanding the theoretical basis of Processability Theory, a theory of second language development that combines insights in the way speakers generate language and store their language knowledge to predict, describe and explain developmental sequences (Pienemann 1998, 2005). The aim of the book is to provide a forum for new perspectives focusing on three intersections: (1) the interface between morpho-syntax and discourse/pragmatics/semantics, (2) constraints on processing and receptive processing and (3) developments in instructed second language learning. Each part also includes a response paper, in which the new perspectives, in terms of the theoretical challenges and/or the empirical results of the preceding chapters are discussed.