This paper describes an analysis of the biped robot domain. Papers in the robotics literature focus on bipeds that walk and run. Typically biped research is centered on the use of one or more models of the motion dynamics. This paper presents three categories of these models: 1) Joint level models 2) leg level models, and 3) body level models. A contribution of this paper is to expose the way these models axe used and to show the simplifying assumptions that are used to employ them to control a biped. The second contribution is to cast the simplifications into a qualitative framework and open the field to the idea of using qualitative reasoning for dynamic robot control.