I discuss recent advances in the study of somatic and embryonic cell cycles. In the frog embryonic cell cycle, cyclin is the only newly synthesized protein required to activate maturation-promoting factor and induce mitosis. Diminishing the rate of cyclin synthesis increases the length of interphase. Cyclin degradation is required for the progression from mitosis to interphase. Comparison of the frog embryonic cell cycle to other cell cycles suggests that all cell cycles will rely on the same closely conserved set of components. However, the component that is rate-limiting for any step in the cell cycle will vary in different cell cycles.