Predominant, if not exclusive, use of the target language has long been considered an important principle of second language (L2) instruction. Previous research has attempted to quantify the amount of the first language (L1) used in the classroom and has explored the purposes or functions of teachers' "lapses" into their students' L1. The present study is a detailed analysis of one teacher's language use during one semester of a university-level Spanish course. The goal is fivefold: to determine the amount of L1 used; to analyze the functions of L1 use; to compare the teacher's perceptions with her actual L1 use; to compare students' perceptions with the teacher's actual L1 use; and to identify motivations or reasons underlying her L1 use. The findings have implications for classroom practice and emphasize the value of self-recording in teacher development. (Contains 4 tables and 3 notes.)