Molecular transformation is commonly offered as a hope to overcome the apparent stagnation in crop yield potential. A basic understanding of the resource limits imposed on crops and the yield hierarchy going from gene expression to harvestable yield leads to a rather negative view that transformations of a few, or even of a complex of genes will result directly in major yield increases. Forty years of biochemical and physiological research illustrate the great difficulty in translating research at the basic level into improvements in crop yield. However, there are a few cases where physiological research has led to improved crop cultivars with increased yield. These successes are instructive in highlighting key elements required to achieve success in developing crop cultivars for increased yield.