This paper reports on the Video Viewing Task (VVT), an assessment designed to measure teachers' developing understanding of two reading comprehension instruction approaches: Reciprocal Teaching (Palincsar, A. S. & Brown, A. L. (1984). Reciprocal teaching of comprehension-fostering and comprehension-monitoring activities. "Cognition and Instruction", 1(2), 117-175) and Questioning the Author (Beck, I. L., McKeown, M. G., Hamilton, R. L., & Kucan, L. (1997). "Questioning the author: An approach for enhancing student engagement with text". Newark, DE: International Reading Association). The VVT consists of video excerpts that teachers view and comment on "during" their viewing. The VVT also includes a series of questions to guide teachers' reflections "after" viewing. Analyses of teachers' comments and reflections in response to the VVT provide evidence that the VVT captures teachers' entering conceptions and is sensitive to changes in teachers' developing understanding following professional development specific to the two instructional approaches. (Contains 7 tables.)