Video shows promise for supporting mathematics teacher candidates’ understanding of student thinking. As a result, we began a design-based research project on using video-based online learning modules as a component of a teacher preparation program to improve candidates’ ability to recognize and make connections between patterns in students' informal and formal reasoning. In this paper, we describe the initial development process of the video case analysis of student thinking (VCAST) modules and then share the results associated with developing our first two modules on functional reasoning. This design-based research project is informed by literature on student thinking and involves a purposeful selection of authentic examples of student reasoning. The results indicate that our process led to an adequate range of student ideas from which to build the modules. Moving forward, the research team is continuing this work by investigating additional cycles of development, improvement, and implementation of these online modules.