This quantitative study investigated the relationship between teacher dispositions, subject content knowledge, pedagogical content knowledge, and reasons that school principals recommend non-renewal of teachers' contracts. Nearly 2,000 school principals in 13 states completed an emailed survey. In deciding whether to non-renew a teacher contract, principals reported that they observed most a lack of pedagogical content knowledge from ineffective teachers and that they prioritized the importance of instructional skills. Ethical issues received greatest importance. Principals identified teacher integrity, honesty, and dependability as important teacher dispositions. The study's findings are important for the planning of teacher and principal professional development initiatives and hiring selection decisions. Knowing how to accurately ascribe variance in student learning has potential for improving student outcomes, particularly with the emphasis on value-added teacher evaluations.