A characteristic feature of the vertebrate body is its segmentation along the anteroposterior axis, as illustrated by the repetition of vertebrae that form the vertebral column. The vertebrae and their associated muscles derive from metameric structures of mesodermal origin, the somites. The segmentation of the body is established by somitogenesis, during which somites form sequentially in a rhythmic fashion from the presomitic mesoderm. This review highlights recent findings that show how dynamic gradients of morphogens and retinoic acid, coupled to a molecular oscillator, drive the formation of somites and link somitogenesis to the elongation of the anteroposterior axis.