Increasing numbers of students with identified special needs and disabilities attending Australian schools have enhanced the expectation that mainstream teachers will demonstrate competence in planning and teaching for an academically diverse range of students. A qualitative action research study explored how teachers at an Australian secondary campus understood and addressed learner diversity in the context of a whole-school effort towards more inclusive practices. Data were collected over a one-year period as the principal researcher, acting as participant/observer, sought to identify how engagement with professional learning influenced teachers' understandings, attitudes and practices related to differentiated instruction. Findings highlight the role of school leaders in providing sustained, contextual professional learning and shifting a school culture towards more inclusive teaching practices.