Summary: ACTIVEMATH is an open web-based learning environment for mathematics. It dynamically generates interactive mathematical courses adapted to the student’s goals, preferences, capabilities, and knowledge. The content authored for ACTIVEMATH is represented in an extended OMDoc which in turn is an extension of the OpenMath XML-language. ACTIVEMATH is the first system that uses OMDoc. It makes use of this knowledge representation in several knowledge management tasks, among them the web-presentation of mathematical text including formulæ, the communication with the integrated mathematical systems, the user-adaptive composition of a course, the dynamic generation of learning suggestions, and the dictionary facility. This article describes the knowledge representation and management in ACTIVEMATH and emphasizes its bias toward mathematics.