The dramatic increases in wheat yields that began in the mid-1930's in the United States will soon begin to level off. The favorable mix of genetics and technology that has characterized this era must build upon an ever higher yield base for the future. At the same time the residue of factors that can lower wheat yields includes a larger proportion of forces not easily shaped or controlled by man. An example is weather. The result is a natural yield ceiling that is already visible and that will impose a limit on future productivity growth.