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In this paper, stable hopping of a one-legged, articulated robot with a flat foot is investigated. The robot has a special feature that before taking off, it goes through an underactuated phase in which the foot rotates about the unactuated toe on the ground. By having an underactuated phase, the robot can perform stable human-like hops with longer hopping distances. To devise a systematic trajectory design methodology for the robot, its dynamics was first derived, including the effects of ground-foot impact and hopping constraints. An optimization procedure is then proposed to plan the feasible actuated trajectories that not only meet performance requirements but also attain optimality with respect to the use of actuation energy. The hopping strategy and the planned trajectories are then verified by simulation and hardware implementation. Experiments indicate that the robot can perform stable hops with different hopping distances on level ground and can successfully hop up and down staircases. [ABSTRACT FROM AUTHOR]