Responding to the scarcity of suitable measures of teacher knowledge, this article reports on a novel assessment approach to measuring teacher knowledge of teaching mathematics. The new approach uses teachers' ability to analyze teaching as a proxy for their teaching knowledge. Video clips of classroom instruction, which respondents were asked to analyze in writing, were used as item prompts. Teacher responses were scored along four dimensions: mathematical content, student thinking, alternative teaching strategies, and overall quality of interpretation. A prototype assessment was developed and its reliability and validity were examined. Respondents' scores were found to be reliable. Positive, moderate correlations between teachers' scores on the video-analysis assessment, a criterion measure of mathematical content knowledge for teaching, and expert ratings provide initial evidence for the criterion-related validity of the video-analysis assessment. Results suggest that teachers' ability to analyze teaching might be reflective of their teaching knowledge. (Contains 1 figure.)