Corpora and corpus-based methods can make a contribution to the study of variability in learner language for two main reasons. One reason is that the study of linguistic variation itself is particularly amenable to quantitative and corpus-based analysis. The corpus, especially when used in combination with metadata about the learners represented and about the situation in which the language was produced, enables the researcher to quantify and compare data in systematic ways. The quantitative corpus results can then be used to verify or falsify claims made in the second language acquisition (SLA) literature or to generate new hypotheses about learner language. Another reason is that the focus on naturally occurring language in corpus work means that the types of learner data studied represent authentic language use. There is much experimental work in SLA, which means that the language analysed is produced in an experimental setting (such as a laboratory), typically solely for the express purpose of linguistic analysis. While there are many good reasons for the experimental elicitation of linguistic data – the complexity of language use is reduced; the language production and variables potentially affecting it can be controlled; the likelihood of capturing relevant types of linguistic output can be maximised – it is also the case that such data simply do not represent the full gamut of authentic language use. Almost inevitably, researchers who study learner corpus data will encounter linguistic variability and will need to account for it. Learner corpus research has paid a great deal of attention to the influence of the mother-tongue background on learner language (see Chapter 15, this volume), but it has tended to neglect other factors that may exert an influence and that may serve to account for some of the variability attested in learner corpora. This chapter will discuss some of these alternative factors and demonstrate how important they can be in language production in general and in foreign/second language production in particular. 2 Core issues Language is not a static phenomenon, but rather varies – sometimes considerably – depending on why it is used, where it is used, by whom it is used, and so on.