This chapter reviews recent theoretical and empirical literature regarding teachers' uses of the target (TL) and first (L1) languages in second (SL) and foreign (FL) language classrooms. Theoretically, the article explores several issues related to teachers' use of the L1 and the TL in the classroom: exposure to TL input, student motivation, cognitive considerations, code-switching, and appropriate teacher use of the L1. A review of recent discourse analysis studies examines how much, when, and why FL and SL teachers use the L1 and TL in their pedagogy. The article also presents findings from studies that have considered teachers' self-reports and teachers' and learners' beliefs and attitudes regarding the use of the L1 and the TL in FL and SL classrooms. The paper concludes with recommendations for future research.