Crop yields are influenced by a complex combination of factors, including weather, seed genetics, soil quality, and production management. Regardless, yields still tend to show a general trend over time, which is referred to as the “trend yield.” There has been much debate over whether improved technology has caused trend yields to change in recent years. Many experts believe recent corn yield increases were caused by advancements in biotechnology for seed genetics. It has been suggested that a lack of research on soybean genetics has led to a plateau in the soybean trend yield. It has been widely accepted that these new trends began in the mid-1990s and that they should be used for estimating future yields. However, weather should be taken into consideration because it can have a large effect on trend yields. The purpose of this paper is to examine whether trend yields in Illinois for corn and soybeans have changed since the mid-1990s. The effect of both weather and technology on yields in Illinois is estimated over the period 1960-2008. The results of this research provide evidence that weather plays a significant and important role in determining yields.