This study used a multiple case study design to explore the occurrence of pivotal teaching moments, teachers' responses to these moments, and teachers' own perceptions of the impact of these moments on their own knowledge development. The participants were six practicing secondary mathematics teachers. The researcher collected data from teacher created lesson plan outlines, observations of the same lesson delivered to two different classes, participant interviews, and teacher reflection journals. The researcher reviewed the lesson plan outlines prior to observations to understand teachers' anticipations. During observations, the researcher recorded observed pivotal teaching moments, corresponding teacher responses to these moments, and instructional changes between the two observed lessons. Interviews allowed the researcher to identify in-the-moment teacher thinking and teachers' motivations for their responses. Teacher reflection journals provided insights related to teachers' classroom actions and learning. The results confirmed and built upon existing classifications of pivotal teaching moments and teachers' responses, while also identifying seven themes related to teacher motivations for their responses. Teachers' perceptions of changes in their own knowledge base occurred for their content knowledge as well as their pedagogical content knowledge. Future research should explore how pivotal teaching moments are created, how teacher-student interactions shape teacher knowledge development, and examine the role of teachers' reflections about their practice in their knowledge development. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]