This article focuses on an experienced high school mathematics teacher who changed her practice without participating in planned interventions aimed at producing this change. It reports on the nature of the teacher's change from her perspective and an interpretation and understanding of the change from the researcher's perspective. It illustrates how meaningful change can occur when the process is initiated and rooted in the teacher's experience based on a tension in self and/or practice that is personal and real to him or her. It discusses three types of change that are possible depending on the level of engagement of the teacher in professional development opportunities: instrumental change, conceptual change, and foundational change.