The climatic history of the Illinois region during Wisconsinan time is interpreted from the character and extent of the glacial deposits and buried soils of the Lake Michigan Lobe. The Illinoian-Sangamonian history of the lobe is briefly discussed. The history is complex; it includes three major glacial episodes during the Illinoian, and a succession of glacial advances and retreats during the Wisconsinan. The major interglacial stages were times of near climatic equilibrium, and the soil profiles indicate temperatures were higher than those of the present. The glacial stages, on the other hand, were characterized by sharply fluctuating climates, with episodes of glaciation alternating with minor intervals of stability, soil formation, and temperatures approaching those of the major interglacial stages.