Producers of agricultural products, policy makers, and consumers alike have a keen interest in what will happen to crop yields in the future. This study attempts to carefully analyze past trends in crop yields in five Corn Belt states, Illinois, Indiana, Iowa, Missouri, and Ohio, and how they have been affected by weather and technological progress over time;State average yields for corn grain, corn silage, soybeans, small grains, and meadow (leguminous hay) are modeled as a system of equations where yields are functions of weather, technological progress, and nitrogen application. Time series data on yields, nitrogen and corn prices, nitrogen application, and weather are collected. Time is used as a proxy variable for technological progress and the models from all five states are estimated using three stage least squares regression. The estimated models do a good job of fitting the 1951-1980 time series data and they illustrate that technological progress and weather are the most important factors that affect yields;The prospects of favorable future weather are analyzed by regressing weather variables from 1930-1980 or dummy variables for each state and for periods of abnormally favorable weather. These models are able to explain only a small portion of the total variance in weather, but they do indicate that the periods during 1942-1952 and 1961-1973 can be characterized as more cool and wet, and generally more favorable to crop yields than average. However, attempts to use this information to project future weather would be ludicrous;The prospects of future yield increases as a result of technological progress are examined by looking at some of the major factors that affect crop yields. It can be seen that technological progress is a large and complex set of interacting conditions, occurrences and activities that cannot be easily modeled, described, or projected. Attempts to project future yields must be based on various assumptions about future technological progress;Yields are projected for the year 2000, using the estimated models, under six different scenarios based on various assumptions about future technological progress, weather, and nitrogen application. In general, with the noted exception of wheat, there is little evidence found in the time series data that would indicate a leveling off of Corn Belt crop yields in the near future.