The creation of psychometrically sound assessments of teacher well-being is critical given the alarmingly high rates of teacher burnout reported among U.S. educators. The present study sought to address this need by developing the Measures of Stressors and Supports for Teachers (MOST), a teacher-report questionnaire designed to assess ecological and psychological factors that affect teachers' professional well-being. To assess structural validity, the MOST was administered to a sample of K-12 educators (N = 218). Methods outlined in Classical Test Theory and exploratory factor analysis were conducted to examine items and assess the factor structure of the MOST. Factor analytic findings yielded a 40-item, nine-factor structure (Parents, Colleagues, School Leadership and Belonging, Classroom Students, Students With Disabilities, Time Pressure, Professional Development, Safety, and Emotional State). Implications for further validation and use of the MOST are discussed.