The study introduces L.S. Shulman,s theory of educational knowledge. It proceeds from “content related knowledge” to the idea of “a knowledge base for teaching”. In discussing the relevance of Shulman for subject didactics, the concept of “a didactic knowledge of contents” becomes central, which is understood as a special amalgam of subject matter knowledge and of educational knowledge and is therefore exclusively in the province of a teacher’s profession. The category of didactic content knowledge is considered the core of the “knowledge base for teaching”, the constitution of which is a necessary prerequisite of the professionalization of teaching.