In order to respond to the growing academic diversity in classrooms, teachers must recognize that their students have different needs and commit to differentiating instruction accordingly; however, the relationship between teachers' willingness and ability to differentiate instruction and principals' attitudes toward differentiation is unknown. In this qualitative study, the principals and faculty at three schools were interviewed and observed over the course of 3 years. The results suggested that principals played a key role in teachers' willingness and ability to differentiate instruction. Principals successful in encouraging teachers to differentiate exhibited the critical support, desire for change, belief that change was possible, and long-term vision of implementation that teachers required in order to effectively differentiate in their classrooms. (Contains 1 table and 1 endnote.)