This work provides an accessible introduction to the field of corpus-based research into learner language. Part one provides the reader with an overview of the field, explaining its links with corpus and applied linguistics, second language acquisition and ELT. It also reviews the software tools which are currently available for analyzing learner language and contains useful examples of how they can be used. Part two contains eight case studies in which computer learner corpora are analyzed for various lexical, discourse and grammatical features. The articles in part three show how CLC-based studies can help improve pedagogical tools: EFL grammars, dictionaries, writing textbooks and electronic tools and implications for classroom methodology are also discussed.