: We are developing computer vision algorithms for sensing important terrain features as an aid to wheelchair navigation, which interpret visual information obtained from images collected by video cameras mounted to the wheelchair. This paper focuses specifically on a novel computer vision algorithm for detecting curbs and other negative obstacles (i.e. anything below the level of the ground, such as holes and drop-offs), which are important and ubiquitous features on and near sidewalks and other walkways. The algorithm we develop extracts as much information as possible from depth information obtained from stereo video cameras (i.e. pairs of cameras mounted close together); other information (e.g. monocular cues such as intensity edges) will be incorporated in the future. We demonstrate experimental results on typical sidewalk scenes.