The appropriate formulation of boundary conditions in the PN-approximation of kinetic equations poses difficulties, both, from an analytical and a numerical point of view. This topic has therefore been subject to intensive discussion since many years. In this talk, we discuss the systematic handling of boundary conditions in stationary and instationary radiative transfer by a variational approach. This leads to well-posed problem on the continuous level and provides a natural way to define appropriate conditions also for PN-approximation. Furthermore, the framework provides a natural approach to designing structure-preserving discretization schemes via Galerkin approximation. In addition, the we comment on the validity of the conditions in high scattering limits and their efficient implementation in numerical methods.