This paper introduces a new corpus resource for language learning research, the Trinity Lancaster Corpus (TLC), which contains 4.2 million words of interaction between L1 and L2 speakers of English. The corpus includes spoken production from over 2,000 L2 speakers from different linguistic and cultural backgrounds at different levels of proficiency engaged in two to four tasks. The paper provides a description of the TLC and places it in the context of current learner corpus development and research. The discussion of practical decisions taken in the construction of the TLC also enables a critical reflection on current methodological issues in corpus construction.