There is a need for research in STEM (science, technology, engineering, and mathematics) teacher education that addresses the challenge of building teachers' pedagogical skills in fostering the development of mathematical reasoning in students. The Common Core State Standards for Mathematics provide teachers with guidance on how to promote mathematical practices that emphasize reasoning and justification through problem solving and that encourage an exploration of viable strategies, through mathematical modeling and facilitating communication in the classroom, to critique mathematical arguments (National Governors Association, 2010). For many teachers, these kinds of mathematical practices may not be what they experienced as learners, and, therefore, it is not clear to them how to engage their students in ways that enact the new Standards of Mathematical Practices. The use of video clips of children thoughtfully engaged in doing mathematics offers a new lens through which to view student learning and brings forth a question that guides the authors' work: Does teacher study of certain videos improve their ability to recognize the variety of forms of reasoning used by the children? They have been conducting research in teacher education using the problem-solving tasks and videos from prior long-term research on the development of mathematical reasoning in students. This work is part of the Video Mosaic Collaborative, which also makes videos, tasks, and related resources available via the Internet (www.videomosaic.org). In this article, the authors report the results of quasi-experimental studies conducted over three years with pre- and in-service teachers. The underlying hypothesis was that a particular video collection can serve as a pedagogical tool for deepening teachers' awareness of how students' mathematical reasoning can emerge naturally through problem solving when appropriate conditions have been established in the learning environment (Maher, 2008). This hypothesis is premised on the notion that teachers' ability to recognize children's reasoning is likely essential for tackling the bigger challenge of subsequent change in teaching practice. Thus, the authors start to address that challenge by investigating their hypothesis about the pedagogical value of certain videos.