What: The evidence for irregular climatic change between warm ( thermos) and cold ( cryos) extremes is reviewed, concentrating on the last two million years, the Quaternary. Who: The suspects in generating this irregular evolution of temperature and ice volume are the climatic subsystems, atmosphere, oceans, ice sheets, bedrock and biota. Processes within each subsystem are briefly described, including major feedback mechanisms that characterize their collective behavior. How: Simple, semi-empirical models are formulated and shown to have irregular behavior with both quasi-periodic and aperiodic components. Model behavior is compared with the evidence, to determine the likeliest set of suspects.