Corpus linguists are increasingly interested in applying their methodological tool box to the various areas of multilingualism. This paper gives an overview of corpus resources and presents three case studies on L2 foreign language learning that employ quantitative methods. The goal is to demonstrate that corpus-linguistic approaches further our understanding of many hot topics in learner language research, including appropriate characterizations of the L1 input and/or target norm; the adequate modeling of the intrinsically complex and highly L1-specific nature of learner language; and the increasingly recognized role of individual variation in the acquisition process. The paper closes with a brief discussion of how these methods can and should be applied to other areas of multilingualism research.