Abstract: Many animals form a number of temporary structures along their anterior-posterior axis during embryonic development – somites. Cells in somites differentiate to form the sclerotome (forms vertebral column), the dermotome (forms dermis) and myotome (forms muscle). During the last three decades, several models have been proposed to explain the formation of somites, including the clock and wavefront model, the reaction-diffusion type model, clock and induction model, clock and trail model etc. Although some of them are satisfactory in the different aspects of somitogenesis, no one can explain the formation and regulation of somites perfectly. But, most models proposed the segmental clock. Expression patterns of c-hairy1, c-hairy2 in chicken, l-fng in chicken and mouse, her1 and Delta C in zebrafish present powerful evidence for the existence of segmental clock.