The Relationship between Second language use and second language learning Here, we look closely at the relationship between using (that is, performing in) an L2, and learning (that is, developing one’s competence in) that same language. We should note first of all, of course, that ‘performing’ in a language not only involves speaking it. Making sense of the language data which we hear around us is an equally essential aspect of performance. It is also obviously necessary to interpret and to process (= analyse) incoming language data in some form for language development to take place. There is thus a consensus that language input of some kind is essential for normal language learning. During the late 1970s and early 1980s, the view was argued by Stephen Krashen and others that input (at the right level of difficulty) was all that was necessary for L2 acquisition to take place. Input, and what learners do with it, has remained a central issue in L2 theorizing ever since. Krashen was unusual in not seeing any central role for language production in his theory of second language acquisition. Most other theoretical viewpoints support in some form the commonsense view that speaking a language is helpful for learning it, though they offer a wide variety of explanations as to why this should be the case. For example, behaviourist learning theory saw regular (oral) practice as helpful in forming correct language ‘habits’. A directly contrasting view to Krashen’s is the so-called Comprehensible Output Hypothesis, argued for by Merrill Swain and colleagues (for example, Izumi, 2003; Swain, 2005).