Reform-based mathematics instruction emphasizes that mathematics is learned through reasoning and sense-making rather than strict memorization and is taught through facilitation rather than telling (NCTM, 1989, 1991, 1995, 2000). Teachers' engagement with student thinking to inform instruction is central to such teaching. Engagement with student thinking involves eliciting and using evidence of student thinking in instruction (NCTM, 2014). This case study explored how three prospective teachers placed at 4th grade engaged with student thinking in mathematics and used this thinking to guide instruction during six weeks of their student teaching experiences. The prospective teachers were placed at the same school and same grade level and were supported through facilitated team meetings in a community of practice (Wenger, 1998) in which they were coached in the pedagogical and content needs of their instructional units that focused on fractions. The study addressed four questions: (1) How might a prospective teacher's engagement with student thinking be characterized? (2) In what ways does context influence a prospective teacher's engagement with student thinking? (3) In what ways does a community of practice structure facilitate prospective teachers' engagement with student thinking in the area of fractions? (4) How does a prospective teacher's engagement with student thinking influence student understanding of fractions in the 4th grade? Findings revealed that the prospective teachers aligned their beliefs and practices with reform-based mathematics teaching practices and engaged with student thinking. Although their community of practice was initially facilitated by a participant-researcher, the prospective teachers voluntarily adopted and extended the community structure outside of the researcher-scheduled meetings. The study highlights the importance of studying and supporting prospective teachers in the context of the student teaching experience in order to help transfer intentions for teaching learned during teacher preparation coursework into practice. The findings offer several contributions to the field, including a suggested model of support to encourage prospective teachers' engagement with student thinking in mathematics and a proposed model for characterizing the nature of such engagement. The models may be used to assess and support prospective teachers in the use of reform-based mathematics practices that focus on student thinking. [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://www.proquest.com/en-US/products/dissertations/individuals.shtml.]