The reiterated structures of the vertebrate axial skeleton, spinal nervous system and body muscle are based on the metameric structure of somites, which are formed in a dynamic morphogenetic process. Somite segmentation requires the activity of a biochemical oscillator known as the somite-segmentation clock. Although the molecular identity of the clock remains unknown, genetic and experimental evidence has accumulated that indicates how the periodicity of somite formation is generated, how the positions of segment borders are determined, and how the rostrocaudal polarity within somite primordia is generated.