School leadership literature suggests that school effectiveness and ongoing improvement rely on school leaders and principals. Despite significant contributions made by principals towards teaching and learning in schools, there have been few studies that explore principals' perceptions of the factors underpinning their successes and achievements. Drawing on one of Senge's disciplines, personal mastery, this research examines principals' perceptions of some processes that enable their successes. A qualitative research project was undertaken in Singapore using in-depth interviews. Analysis was informed by movement between theoretical dimensions and emerging themes from participants' discussions of their leadership experiences. The findings suggest that personal mastery has a positive influence upon principals and their ability to demonstrate effective school management. (Contains 2 tables.)