In this study, I worked with five secondary mathematics preservice teachers (PSTs) in their final year of their undergraduate teacher education program at a large, midwestern university. I examined written lesson plans, observed enacted lessons, and interviewed PSTs about planning and teaching. The purpose of this study was to better understand how their planning practices influenced their classroom discourse practices, particularly when responding to students' errors and uncertainties. Despite using a robust planning framework, 64% of anticipated student thinking was instrumental (Skemp, 1977) or low level, focused on facts or procedures (compared to relational, high level, or conceptual thinking). The data strongly showed that PSTs were challenged to anticipate student thinking and were likely to over- or underestimate students thinking. Additionally, there were considerable differences in the quality and quantity of PSTs' planned and enacted discourse, particularly in response to errors and uncertainties. Forty-one percent of PSTs' planned discourse moves were low level, 50% of their enacted moves were low level and 58% of their responses to errors and uncertainties also were. However, their high level moves only declined from planning to enactment, not from enacted to responding to errors and uncertainties. PSTs identified four challenges in maintaining high cognitive demand: time management, communicating the purpose of the lesson, scaffolding, and in-the-moment decision making. These results align with prior studies that indicate PSTs struggle to plan for student thinking and need support, but with support can learn to engage in productive discourse practices, particularly in response to errors and uncertainties. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]