Technical Abstract: Radiation exchange at the surface plays a critical role in the surface energy balance, plant microclimate, and plant growth. The ability to simulate the surface energy balance and the microclimate within the plant canopy is contingent upon simulation of the surface radiation exchange. A validation and modification exercise was conducted of the Simultaneous Heat and Water (SHAW) model for simulating the surface radiation exchange over and within wheat, maize and soybean plant canopies using data collected at Yucheng, in the North China Plain and near Ames, Iowa. Methodologies were developed for simulating short-wave and long-wave radiation fluxes within a multi-species, multiple layer plant canopy. Although the original SHAW model slightly underpredicted reflected solar radiation, one would conclude that the simulations were quite reasonable if within-canopy measurements were not available. However, within canopy short-wave radiation was seriously underestimated by the original SHAW model. Improvements were made to the SHAW model for simulating radiation exchange within the plant canopy. Modification to the SHAW model improved both above canopy reflected radiation and within-canopy radiation simulations. Alternative approaches to simulating canopy transmissivity to diffuse radiation had a minor influence on simulated short-wave radiation, but made almost no difference in simulated long-wave radiation or evapotranspiration.