Humans are the only extant mammals that are obligate bipeds. This unique form of locomotion frees the hands from locomotor responsibilities and is energetically economical, but is not without its costs. The fossil record clearly indicates that habitual bipedalism had evolved approximately 4 Ma; however, there is tantalizing evidence that upright walking may go farther back into the Pliocene, or even into the Miocene. Recent fossils demonstrate that the evolution of bipedalism was not a simple, linear process, but that there was diversity in bipedal locomotion among Plio-Pleistocene australopiths, and even among the genus Homo. Despite gains in our understanding of bipedal evolution, many questions remain, including when exactly our ancestors took their first steps, the bodily form from which bipedalism evolved, and why this unusual form of locomotion was selectively advantageous.