Technical Abstract: Current photosynthesis is directly or indirectly the source of all of our food and fiber and is increasingly looked on as a potential source of renewable fuels. Increasing world population, improving economic status of portions of the developing world, and limited scope for recruitment of additional arable land, forecast that a doubling of productivity on a land area basis will be needed to meet agricultural demand before the end of this century. A starting point for evaluating the global potential to meet this goal is to critically evaluate the maximum efficiency of photosynthetic solar energy conversion that is possible; i.e., the upper limit of what theoretically could be achieved in managed ecosystems. Based on the current detailed understanding of the component mechanisms of plant photosynthesis our analyses show that, before accounting for mitochondrial respiration, the maximum efficiency possible for conversion of solar energy to biomass is 6.5% for C3 photosynthesis operating at 30C and today's level of atmospheric [CO2]. Whereas the upper limit of solar energy conversion efficiency of C4 photosynthesis is substantially higher at 8.5%, our analysis shows that this advantage over C3 will disappear later this century as atmospheric [CO2] nears 700 ppm.