Transiting planets provide a unique opportunity to study the atmospheres of extra-solar planets. Radiative hydrodynamical models of the atmosphere provide a crucial link between the physical characteristics of the atmosphere and the observed properties. Here I present results from 3D simulations which solve the full Navier-Stokes equations coupled to a flux-limited diffusion treatment of radiation transfer. Variations in opacity amongst models leads to a variation in the temperature differential across the planet, while atmospheric dynamics becomes much more variable at longer orbital periods. I also present 3D radiative simulations illustrating the importance of distinguishing between optical and infrared opacities.