Technical Abstract: The yield potential (Yp) of a genotype of a grain crop, is the mass of seed per unit ground area that is obtained under optimum growing conditions in the absence of weeds, pests and diseases. It is determined by the product of the efficiency of light capture, the efficiency with which the captured light is converted to biomass, and proportion of biomass partitioned into grain. The large increases in the potential yield of the world¿s major grain crops over the past 50 years have been achieved largely by improving the proportion of biomass in grain and efficiency of light interception. This has included selection of genotypes able to benefit from increased nitrogen fertilization to improve. Overall, increases have been achieved with little or no improvement of the efficiency of use of intercepted light and is primarily determined by photosynthesis. In modern, cultivars are now close to their theoretical maxima, and therefore photosynthesis is the only major route for further improvement of yield potential. A number of influential past studies have questioned the value of increased leaf photosynthesis as a means to improving potential yield. These conclusions were based on comparing plants, genetically different in many respects besides photosynthetic capacity. The plethora of research on the effects of elevated CO2 on crop photosynthesis now shows that within the same genotype, increase in leaf photosynthesis is very closely linked to increased yield. Are there opportunities to achieve similar increases by genetic manipulation? Six potential routes of increasing by improving photosynthetic efficiency are explored, from altering canopy architecture to improved regeneration of the acceptor molecule for CO2. While it is uncertain whether some of these changes could ever be realized or would actually result in increased photosynthesis, others could clearly be realized on a short time scale. Collectively these changes could improve, therefore, yield potential by 50%. Since some of these changes could be achieved by transgenic technology, the time by which they could also be realized in material for the production of commercial cultivars will be considerably less than via conventional breeding and potentially within a 10-15 year time horizon.