While education is fundamentally concerned with driving change, generating robust evidence of significant improvements in student achievement, equity, and teacher retention remains notoriously difficult. This chapter presents a comprehensive program of research designed to achieve such transformative outcomes by strengthening teacher capacity through professional development—specifically via an approach known as Quality Teaching Rounds (QTR). QTR deepens teachers’ understanding of effective pedagogy through reciprocal lesson observations, guided by a detailed framework that offers both conceptual clarity and practical support. Importantly, the approach also addresses the power dynamics that often impede changes in everyday teaching practice, instead fostering a powerful form of collegiality. The research program underpinning this chapter is extensive, comprising five randomised controlled trials, qualitative studies, case analyses, longitudinal research, and whole-school reform initiatives. Together, these efforts have produced an unparalleled body of evidence demonstrating gains in student achievement, enhanced equity, improved teacher morale and self-efficacy, and sustained whole-school change. The chapter begins with an overview of the QTR initiative and its distinctive features as a collaborative model of professional development. It then presents findings from two decades of research into the impact of QTR. The significant effects on teachers, teaching, student achievement and equity underscore the importance of shared pedagogical language, sustained time for collaboration, and a climate of trust in enabling profound educational transformation.