The study examines the use of video clips from teachers’ own classrooms as a resource for investigating student mathematical thinking. Three dimensions for characterizing video clips of student mathematical thinking are introduced: the extent to which a clip provides windows into student thinking, the depth of thinking shown, and the clarity of the thinking. 26 video clips were rated as being low, medium, or high on each dimension. The analysis suggests that the relationship between the video clip dimensions is most important in predicting whether a video clip will support in-depth conversations of student thinking on the part of teachers.