This chapter considers how animals use extracellular signals to control the survival, growth, and division of their cells. It describes the complex system of regulatory proteins called the cell-cycle control system, which orders and coordinates these events to ensure that they occur in the correct sequence. The chapter discusses in detail the major stages of the cell cycle, in which the chromosomes are duplicated and then segregated into the two daughter cells. The details of the cell cycle vary from organism to organism and at different times in an individual organism's life. The cell-cycle control system therefore plays a central part in the regulation of cell numbers in the tissues of the body; if the control system malfunctions such that cell division is excessive, cancer can result. Some extracellular signal molecules inhibit rather than promote cell survival, cell growth, or cell division.