Solar radiation is not only the main source of energy for life on Earth, but it is also the natural source of light for the optical acquisition of information, such as for vision and photography . Natural sunlight provides the illumination required for taking hemispherical photographs in forests. In turn, such photographs can then be interpreted to provide a description of the plant canopy and its interactions with light, or more generally, its radiation regime. The techniques used for both the acquisition and the analysis of hemispherical photographs are based on the theory of radiation in plant canopies. The goal of this chapter is to present the theoretical foundations describing how solar radiation reaches Earth’s surface and interacts with plant canopies. Different sensors for measuring radiation are described, and the principles of hemispherical photography are reviewed. Finally, a comparison of the techniques used to assess the radiation regime of forest canopies is presented.