Extracting meaningful 3D human motion information from video sequences is of interest for applications like intelligent humancomputer interfaces, biometrics, video browsing and indexing, virtual reality or video surveillance. Analyzing videos of humans in unconstrained environments is an open and currently active research problem, facing outstanding scientific and computational challenges. The proportions of the human body vary largely across individuals, due to gender, age, weight or race. Aside from this variability, any single human body has many degrees of freedom due to articulation and the individual limbs are deformable due to moving muscle and clothing. Finally, real-world events involve multiple interacting humans occluded by each other or by other objects and the scene conditions may also vary due to camera motion or lighting changes. All these factors make appropriate models of human structure, motion and action difficult to construct and difficult to estimate from images. ...