For many decades, foreign language teaching has been dominated by the principle that teachers should use only the target language (TL) and avoid using the mother tongue (L1) except as a last resort. However, reports show that teachers make extensive use of the L1. This paper illustrates this discrepancy and considers some main reasons for it. It suggests a framework of principles for balancing L1 and TL use in the classroom. Finally, it reviews some strategies available to teachers who wish to make maximal use of the TL without denying the potential of the L1 to support foreign language learning. Copyright © Cambridge University Press 2009.