Abstract: OpenGL and its extensions provide access to advanced per-pixel operations available in the rasterization stage and in the frame buffer hardware of modern graphics workstations. With these mechanisms, completely new rendering algorithms can be designed and implemented in a very particular way. In this paper we extend the idea of extensively using graphics hardware for the rendering of volumetric data sets in various ways. First, we introduce the concept of clipping geometries by means of stencil buffer operations, and we exploit pixel textures for the mapping of volume data to spherical domains. We show ways to use 3D textures for the rendering of lighted and shaded iso-surfaces in real-time without extracting any polygonal representation. Second, we demonstrate that even for volume data on unstructured grids, where only software solutions exist up to now, both methods, iso-surface extraction and direct volume rendering, can be accelerated to new rates of interactivity by simple polygon drawing and frame buffer operations.