An exoskeleton worn by a human user consists of a rigid pelvic harness, worn about the waist of the user, and exoskeleton leg structures, each of which extends downwardly alongside one of the human user's legs. The leg structures include hip, knee, and ankle joints connected by adjustable length thigh and shin members. The hip joint that attaches the thigh structure to the pelvic harness includes a passive spring or an active actuator to assist in lifting the exoskeleton and the human user with respect to the ground surface upon which the user is walking and to propel the exoskeleton and human user forward. A controllable damper operatively arrests the movement of the knee joint at controllable times during the walking cycle and a spring located at the ankle and foot member stores and releases energy during walking.