Teachers' efficacy beliefs are critical to improving student learning, but we have yet to fully understand how these beliefs develop. The prevailing model of teachers' efficacy development emerges from cognitive theories, but sociocultural theories may add insights to modeling the impact of teachers' school contexts. This case study takes a "communities of practice" approach to explore the connections between teachers' evidence-based decision-making practices and their efficacy beliefs. Findings, based on interviews with four teachers, indicate that teachers co-construct their efficacy beliefs in shared practices, suggesting the usefulness of communities of practice theory to more fully understand teachers' efficacy belief development.