This chapter describes how corpora and corpus methods can be used in the analysis of learner language and applied to language testing. The chapter first describes three types of corpora relevant for investigations of learner language: corpora of L1 and expert language, L2 (learner) corpora, and corpora representing language produced under exam conditions. Next, the chapter provides a brief overview of corpus methods most relevant for corpus-based research with application for assessment of second language ability. Finally, direct applications of corpora for development and validation of tests and research instruments in SLA are illustrated by examples from recent research.