Much of our current understanding of the cell cycle involves analyses of its induction in quiescent cells. To better understand the control of cell cycle propagation and termination, studies have been performed in actively cycling cultures using time-lapse photography and quantitative image analysis. These studies reveal a highly ordered sequence of events required for promotion of continued proliferation. The decision to continue cell cycle progression takes place in G2 phase, when cellular Ras induces the elevation of cyclin D1 levels. These levels are maintained through G1 phase and are required for the initiation of S phase, at which time cyclin D1 levels are automatically reduced to low levels. The reduction of cyclin D1 to low levels during S phase is required for DNA synthesis, and forces the cell to induce high cyclin D1 levels once again when it enters G2 phase. In this way, cyclin D1 is proposed to serve as an active switch in the regulation of continued cell cycle progression.