Human-like running is a natural dynamic mode of a simple mechanical biped. Such a machine consists of two telescoping legs with linear springs, connected by a hip joint with a torsional spring. It will run passively; no pattern of forcing is required to generate the gait. With careful design its energy consumption can approach zero, but in any case the passive cycle can be `pumped' by various means to sustain running over a range of speeds and slopes. Passive running can also be realized over a wide range of mechanical design parameters. Some parameter sets produce cycles that are inherently stable; otherwise the mode can be actively stabilized by a simple control law. Thus the passive running model offers an effective foundation for design of practical running machines, and also provides an insight into the physics of human locomotion.