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Robot walking, while appealing for its resemblance to human motion, is not an obvious choice when both economy and versatility are desired. Wheeled vehicles are surprisingly capable on different terrains and are nearly unbeatable in terms of economy. In specialized situations, legged locomotion may become preferable. But legged locomotion entails inertial and other energetic costs that do not appear in wheeled machines. The force and work requirements of legged locomotion also only appear energetically economical when considering the unique features of the human body and human muscle. The attainment of high economy in a legged robot requires either actuators similar to humans' or discontinuous nonlinear mechanisms that can reduce energetic losses to support a load. The attainment of high versatility indicates that the ZMP is likely to remain applicable, unless serious advances are made in other control theoretical approaches.