Yield gaps are the difference between actual yield at the field and potential yield. Potential yield represents the yield without any limitation from weather or nutrients. It is difficult to obtain long-term yield records from producers on a field scale and a surrogate for field scale is to use county or cropping district yields. These data allow for a data record of sufficient length to evaluate trends in technology and the impact of the weather events in specific growing seasons. To quantify yield gaps, we have used the attainable yield, defined as the upper frontier of the yield observations and computed the yield gap as the difference between attainable and actual yield at a specific geographic region. We have conducted this analysis for county, cropping district and state level yields in the United States and province level yields in Canada over the period from 1950 through the present. There is variation among years because of weather conditions during the growing season; however, the major variable affecting the yield gap was the precipitation during the grainfilling period. The next variable affecting the yield gap was the occurrence of frost during flowering or grain-filling period and this was infrequent. Yield gaps vary by geographic region and range from an average of 10% to 30% of the attainable yield. Understanding the yield gap provides guidance on how we can begin to use the genetics x environment x management (GxExM) concept to evaluate strategies to enhance production around the world.