This chapter reviews the observations of the radiative fluxes inside and outside the Venusian atmosphere, along with the available data about the planetary energy balance and the distribution of sources and sinks of radiative energy. We also briefly address the role of the heat budget on the atmospheric temperature structure, global circulation, thermodynamics, climate and evolution. We compare the main features of radiative balance on the terrestrial planets, and provide a general description of the radiative-convective equilibrium models used to study their atmospheres. We describe the physics of the greenhouse effect as it applies to the evolution of the Venusian climate, concluding with a summary of outstanding open issues.