This book explores the way in which language processing mechanisms shape the course of language development, adding a psychological component to the search for a theory of second language acquisition. The core of the book is Pienemann’s Processability Theory which spells out which second language forms are processable at which developmental stage. The theory is based on recent research into language processing and is formalised within Lexical-Functional Grammar. The predictions of the theory are applied to the second language development of English, German, Japanese and Swedish. The theory is also tested in on-line experiments. In addition, Processability Theory has major implications for interlanguage variation (including task variation) and age-related differences in language acquisition.