The cell cycle consists of a set of highly ordered events that result in the duplication and division of a cell. This process requires the synthesis of a new copy of DNA, segregation of chromosomes, mitosis, and apportionment of the cellular contents. Multiple extracellular signals control entry and exit from the cell cycle to coordinate normal cell growth and to avoid uncontrolled cell proliferation. Various steps in the progression of the cell cycle are regulated rigorously to allow surveillance of the cycle and to avoid errors in DNA replication. This chapter reviews the molecular basis for control of the cell cycle. Much progress has come from studies of yeast, clams, and other model systems. These mechanisms are largely conserved in mammalian systems, which are the main focus of this chapter.