Understanding the properties and evolutionary stage of protoplanetary disks requires to be able to derive physical quantities from multi-wavelength and multi-technique observations of these disks. Because of the complexity of the radiative transfer, we must rely on sophisticated numerical codes to do so. In this chapter, we present the radiative transfer problem in dusty media and discuss why this represents a difficult numerical problem to solve. We briefly describe the various methods that can be used to solve the radiative transfer equation and discuss their relative merits and drawbacks.