Radiative transfer calculations are the main tool and also a main challenge to analyze the radiation we are receiving from regions where stars are formed. Most of the young stars are deeply embedded in clouds containing molecular and atomic gas as well as dust and therefore are invisible in the UV to NIR wavelengths range. In this review, the basic aspects of calculating the radiation field in a dusty medium are described. On the basis of the 3D radiation transport equation, the physical conditions in star-forming regions and the corresponding integro-differential equation structure are discussed. I will scetch the different solution methods with emphasis on the key issues resolution and error control. For ray-tracing on adaptively refined grids, the choice of appropriate refinement criteria, self-consistent energy conservation schemes, the handling of boundary conditions, and benchmarking are described. The current status and future improvements are illustrated using several recent applications of radiative transfer calculations for star formation regions:
-The spatial and thermal 3D structure of a dense molecular cloud core in Rho Oph has been determined by a detailed inverse radiative transfer modeling of MIR and mm images.
-Using 3D Smoothed Particle Hydrodynamics simulations of an evolving and later collapsing pre-stellar core along with a 3D Radiative Transfer analysis, it is shown that commonly applied 1D modeling based on column density interpretation of images does not produce reliable structural information.
-The analysis of multi-wavelengths images of an accretion disk candidate around a massive star seen in silhuette can reveal if massive stars form by disks or by star merging.
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