Increasing the potential yield of wheat is a major thrust of CIMMYT's Wheat Program. Farmer's wheat yields are linearly related to the yield potential of new varieties. At the same time, breeders should ideally be able to assess a new line's yield potential as early as possible after a cross has been made. Unfortunately, present breeding techniques do not allow confident assessment of grain yield in early generations. The work covered in this Wheat Special Report is a contribution to our understanding of the factors limiting grain yield in wheat. It is also a test of Donald's ideas of adaptation to communal growth. We hope that the results and discussion shed some light on our main objectives, i.e., selecting high yielding wheats in early generations through a better physiological understanding and increasing the yield potential of wheat.