How do mammalian cells make the choice between proliferation (cell-cycle progression) and quiescence (cell-cycle exit)? Correct execution of the proliferation-quiescence decision is important in many biological settings, from developmental processes to adult tissue homeostasis, and dysregulation of this decision occurs in nearly all cancer types. Yet despite clear medical relevance, we have a limited understanding of the inputs that control the choice between proliferation and quiescence, when during the cell cycle cells integrate the presence or absence of these inputs, and how cancer cells rapidly rewire this program to escape quiescence induced by targeted cancer therapies. To tackle these issues, the Spencer Lab has developed cutting-edge cell biology methods, including novel fluorescent biosensors, multi-day time-lapse microscopy, and automated single-cell tracking, providing us a longitudinal, multigenerational view of cell-cycle behavior. We also pioneered the automated pairing of live-cell microscopy with subsequent fixed-cell staining, which allows us to match each cell’s current molecular state with its history. Our work on the proliferation-quiescence decision has revealed remarkable heterogeneity in cell behavior even amongst cells in a uniform environment, as described below.