Most crops are well adapted to the prevailing climate. Nevertheless, weather remains the most significant factor in successful crop production. Weather-wise management of production and marketing can greatly reduce the risk of crop loss and of fmancial failure in crop production. The cyclic nature of midwest weather, the relationship of potential yield to soil moisture, and the effects of temperature on the development of crops and crop pests should be understood by the farmer and by the farm service representative. Com yield in Iowa and Illinois improved slowly during the first half of the 20th century. There was considerable yield variation from year to year because of weather and pests, but improvement was, overall, consistent. From 1955 to 1971, the yield trend improved rapidly and year-to-year variability was small. Some attributed the improvements to land management and some to greatly improved hybrids. Even the consistency from year to year was thought by some to indicate that modem hybrids were not greatly affected by the year-to-year variations in the weather. Whatever the reason, consistent yields ended with the onset of wide-spread com disease in the early '70s and the spread and success of the disease itself was weather oriented. Year-to-year variability since 1972 has been as great as it was during the years before 1955.