The purpose of this article is to explore the usefulness of a video-based tool for measuring teachers' mathematical knowledge for teaching. Unique to this tool is the use of a video featuring a mathematical disagreement that occurred in an elementary classroom. The authors define mathematical disagreements as instances in which students challenge each other's mathematical ideas (Barlow & McCrory, 2011). The authors anticipated that by having teachers view video of a mathematical disagreement and then respond to questions, they could effectively access their mathematical knowledge for teaching. This use of a video featuring a mathematical disagreement proved to be an effective way to access the participants' knowledge.