Studies on school restructuring and the leadership role of the principal in this process suggest that what has been the traditional leadership approach of the principal appears to be changing in relation to the substantial changes and school-wide reforms that are continually taking place in schools today. These school reform initiatives necessitate new and creative ways of thinking about our concept of educational leadership and its various approaches. It also became clear from the literature on leadership that a person's assumption of various types of knowledge influence his or her leadership approach. The purpose of this study, based on a quantitative empirical study in selected South African schools, is to identify this impact of principals' assumptions of knowledge on their leadership approaches. A total of 100 questionnaires with open-ended questions were sent electronically to school principals of randomly selected schools to assess the link between principals' assumptions about the nature of knowledge and principal leadership. The "t"-test, Lambda test and an Analysis of Variances test were used to analyse the data. The findings confirm the relationship between core epistemological beliefs (EBs) and beliefs about leadership practices among South African school principals and provide substantial justification for using EBs in the study of school leadership.