Web-based digital video tools enable learners to access video sources in constructive ways. To leverage these affordances teachers need to integrate their knowledge of a technology with their professional knowledge about teaching. We suggest that this is a cognitive process, which is strongly connected to a teacher's mental model of the tool's affordances. First we elaborate the theoretical integration of the notion of mental models and the Technological Pedagogical Content Knowledge (TPCK) framework. Then we report on a study where we investigated pedagogical knowledge in a sample of German pre-service teachers as a predictor for their mental models of YouTube and how these affect lesson plans for instructional use of this technology. We describe the active mental models of YouTube and present quantitative analyses suggesting mental models as mediators for the influence of pedagogical knowledge on participants' lesson planning. Results are discussed with regard to theoretical and research implications. (Contains 3 tables and 6 figures.)