Most likely produced as thermal emission by dust grains, infrared emission from young stars and active galaxies reveals information about the internal energy source, the spatial distribution of the surrounding matter and its dynamical evolution. In this way, radiative transfer works as a bridge between HD/MHD accretion disk theory and observations. For young stellar objects and active galactic nuclei, the source geometry is of crucial importance for the modelling the emerging radiation. Observations indicate rotationally symmetric accretion disks, tori, or more complicated density structures around multiple sources, making multidimensional calculations necessary.