Biped locomotion is a type of movement in which the mover makes use of the gravity skillfully. The purpose of this paper is to investigate the dynamics of biped locomotion from the viewpoint of both classical mechanics and modern control theory. To this end, a mathematical model with moderate degrees of freedom is proposed, which is derived as a model of five-link locomotion expressing the essence of human biped locomotion. The structural features of the model are discussed from the viewpoint of freedom of dynamics and controllability. It is then shown that some specific dynamical features of the locomotion model can be characterized in terms of angular momentum. It is finally shown that the ‘gait stability’, which is a special type of stability inherent to biped locomotion, is realized by a proposed control which satisfies the final condition of a swinging leg.