In this article, we describe the development and application of a videotape assessment in high school physics. The assessment was codeveloped by a team of expert high school physics teachers and college faculty. The rationale for the assessment and a description of its development are provided. The administration of the assessment and the scoring procedures applied to students' responses are described. The scoring system had high interrater reliability and high internal consistency. Furthermore, it could discriminate between students in regular physics classes and students in honors physics classes. The provision of feedback to students during the assessment helped them to improve their performance significantly.