The acquisition of human motion data is of major importance for creating interactive virtual environments, intelligent user interfaces, and realistic computer animations. Today?s performance of off-the-shelf computer hardware enables marker-free non-intrusive optical tracking of the human body. In addition, recent research shows that it is possible to efficiently acquire and render volumetric scene representations in real-time. This paper describes a system to capture human motion at interactive frame rates without the use of markers or scene-intruding devices. Instead, 2D computer vision and 3D volumetric scene reconstruction algorithms are applied directly to the image data. A person is recorded by multiple synchronized cameras, and a multi-layer hierarchical kinematic skeleton is fitted to each frame in a two-stage process. We present results with a prototype system running on two PCs.