The main aim of this special issue of Instructed Second Language Acquisition (ISLA) is to address and illustrate the relationship between second-language instruction, inspired by processability theory, and second-language learning, as a contribution to disclosing, however partially, the complex relationship between theories of SLA and classroom practice. Instructed SLA, according to Long (2017), is a subfield of SLA, and its research findings are 'often relevant for LT and classroom processes, certainly, but are distinguishable from them' (p. 8).Long's assertion echoes calls from the late sixties and early seventies, when SLA was not yet called SLA, and when there was a feeling, with Corder (1967) at the forefront, that looking at what language learners did, focusing especially on errors, would be of interest to researchers. Corder assumed learners possessed a kind of in-built syllabus and believed that such research might turn out to be beneficial for teachers of languages. Ever since, however, the relationship between research and teaching has been a rather elusive one. Reacting to the behaviourist approaches to language teaching prevalent at the time Dulay and Burt (1973) responded in the negative to their own question whether children should be taught syntax. These and other researchers, most notably Krashen (Krashen and Terrell 1983) came to assume the existence of a 'natural order' of language development, regardless of the learner's age or first-language background, then the role of the teacher was that of providing comprehensible input