Understanding complex subjects and theories is a difficult task for human beings. With the ISO Standardization of XML Topic Maps (XTM) a new solution is found to describe complex subjects. A suitable visualization of such complex subjects based on Topic Maps supports the mediation of knowledge and enables students to understand the complex theories in a intuitive way. At the Fraunhofer-Institute for Computer Graphics a 3D Topic Map Viewer was developed to match this special requirements in e-learning environments. The complex subjects are described as knowledge domains based on the Topic Map paradigm. The application generates automatically an intuitive 3D graph presentation of the knowledge domain. Various interactions for navigation and selection support the students learning processes by touch and play with the knowledge domain representing a complex subject.