In human walking, the center of mass moves along an approximately pendular arc dictated by the stance leg. Each transition to a new stance leg requires negative mechanical work to redirect the center of mass velocity from one inverted pendulum arc to the next and an equal amount of positive work to restore the energy lost (step-to-step transition costs). Mathematical models of walking predict that less mechanical work is required during transitions if the positive and negative work are performed simultaneously and that the amount of work performed during transitions depends strongly upon speed, step length and step width. Further, any mechanical work performed by muscles will require metabolic energy. To test these predictions, I first derived a new measure of mechanical work to estimate the work performed on the center of mass by individual limbs and then applied it to human walking at a range of speeds, step lengths, and step widths. As predicted, the legs have distinct functions during the transition between steps—the trailing acts as a motor, performing positive work, while simultaneously the leading leg acts as a brake, performing an approximately equal amount of negative work. Also as predicted, step-to-step transition costs depend strongly on step length and step width—walking with long steps or wide steps requires considerable mechanical work. Further, the mechanical work of step-to-step transitions exacts a proportional metabolic cost with efficiencies similar to those found in slope walking and isolated muscle experiments. I also found that people prefer a step width that minimizes metabolic cost. The metabolic minimum appears to exist because of a tradeoff between increasing transition costs at wide widths and an increasing cost to moving the swing leg laterally to avoid the stance leg at narrow widths. The preferred width is relatively narrow— transition costs due to step width likely contribute relatively little to the metabolic cost of normal walking. In contrast, people prefer to walk with relatively long step lengths—step-to-step transition costs due to step length appear to comprise a substantial fraction of the metabolic cost of normal walking.