The purpose of the study was to examine the differences among significant changes in the practice of individual teachers. Seventeen US teachers were interviewed about the most successful change in their teaching career. The differences in teacher change were based primarily on the source and impact of the change. The sources of change were divided into two categories: other-initiated or self-initiated. The level of impact was determined in three ways: by whether teachers described a new practice, a new direction, or a change in their beliefs. By organizing according to both the source and impact of the change, four types of change were identified: "Response to School Initiatives," "Professional Development Initiatives," "Student-Centered Changes," and "Self-Designed Changes." Self-Designed changes were further distinguished based on the time required for the change process. Implications for developing new strategies to foster teacher development are discussed.