Teachers are the primary implementers of social and emotional learning (SEL) programs. Their beliefs about SEL likely influence program delivery, evaluation, and outcomes. A simple tool for measuring these beliefs could be used by school administrators to determine school readiness for SEL programming and by researchers to better understand teacher variables that impact implementation fidelity and program outcomes. In a two-phase study, we developed and then validated a parsimonious measure of teachers' beliefs about SEL. In Phase 1, survey items were administered to 935 teachers and subjected to both exploratory and confirmatory factor analysis, resulting in three reliable scales pertaining to teachers' "comfort" with teaching SEL, "commitment" to learning about SEL, and perceptions about whether their school "culture" supports SEL. Phase 2 provided evidence for the concurrent and predictive validity of the scales with a subsample of teachers implementing an SEL program as part of a randomized controlled trial. The discussion focuses on the value of measuring teachers' beliefs about SEL from both researcher and practitioner perspectives. (Contains 1 note, 3 tables, and 1 figure.)