The COACTIV video study is part of the COACTIV research program in which secondary mathematics teachers whose students participated in PISA 03/04 were examined, with respect to their professional knowledge, motivational orientations, beliefs, and self-regulation. In the video study, 284 German secondary mathematics teachers were asked to specify how they would continue lessons shown in three short video clips that all ended at “educationally crucial” points. From the teachers’ written responses, which were coded by two independent evaluators according to five dimensions of high-quality teaching, their “situated reaction competency” (SRC) was inferred. Results relating to differences in school type (e.g., teachers from the German academic track performing better) and the relationship of SRC to other teacher characteristics (e.g., SRC was positively related to constructivist beliefs), as well as its impact on specific aspects of instructional quality, indicated the validity of the instrument.