There is now strong evidence that yield potential in rice (Oryza sativa L.) is becoming limited by ‘source’ capacity, i.e. photosynthetic capacity or efficiency, and hence the ability to fill the large number of grain ‘sinks’ produced in modern varieties. One solution to this problem is to introduce a more efficient, higher capacity photosynthetic mechanism to rice, the C₄ pathway. A major challenge is identifying and engineering the genes necessary to install C₄ photosynthesis in rice. Recently, an international research consortium was established to achieve this aim. Central to the aims of this project is phenotyping large populations of rice and sorghum (Sorghum bicolor L.) mutants for ‘C₄-ness’ to identify C₃ plants that have acquired C₄ characteristics or revertant C₄ plants that have lost them. This paper describes a variety of plant phenomics approaches to identify these plants and the genes responsible, based on our detailed physiological knowledge of C₄ photosynthesis. Strategies to asses the physiological effects of the installation of components of the C₄ pathway in rice are also presented.