Vertebrate somitogenesis has been shown to be associated with a molecular oscillator, the segmentation clock, whose periodicity matches that of the process of somitogenesis. The existence of such a clock in presomitic mesoderm (PSM) cells was originally proposed in theoretical models such as the ‘clock and wavefront’. Molecular evidence for the existence of this clock in vertebrates has been obtained on the basis of the periodic expression of several genes, most of which are related to the Notch signalling pathway. These genes are expressed in a dynamic sequence which appears as a wave sweeping caudo-rostrally along the whole PSM once during each somite formation. Notch-pathway mouse and fish mutants lose the dynamic expression of the cycling genes, indicating that Notch signalling is required for their periodic expression, or is required to coordinate the oscillations between PSM cells. Therefore Notch signalling is either part of the mechanism of the oscillator itself or acts as a cofactor required for cycling gene expression. A further potentially important role for the segmentation clock is to periodically activate Notch signalling in the rostral presomitic mesoderm, thereby generating the periodic formation of somite boundaries.