This book investigates first language (L1) and second language (L2) use in Chinese university classrooms, focusing on the experiences of four Chinese EFL teachers who were teaching non-English major students at four different proficiency levels. It examines these four teachers' actual use of L1 and L2, including the distribution of their L1 and L2 use, the circumstances, functions and grammatical patterns of their language use, and their language use across different frames of classroom discourse. It also explores their attitudes and beliefs regarding this issue in depth, as well as their own perceptions of and reasons for their language use and possible influencing factors. Through its detailed analysis of the teachers' language use, as well as their respective beliefs and decision-making techniques, this book contributes to L2 teachers' professional development and L2 teaching in general, especially with regard to establishing a pedagogically principled approach to L1 and L2 use.