Gamma-ray bursts (GRBs) represent powerful, time variable sources that illuminate the interstellar medium. Studies of absorption lines in the spectra of GRB afterglows can provide valuable information about the properties of their circumstellar environments and host galaxies. The calculation of photon transport through absorbing material around time dependent sources is a challenging problem in radiative transfer, due to the competition between timescales that characterize the variability of the emission, the time between radiative interactions, and the light crossing time for the medium. I will discuss a new method for solving the time dependent radiative transfer problem for multi-level atoms and ions in dynamic media that is irradiated by an external source. I will show that a careful treatment of radiative transfer in GRB absorbing environments can yield very different results than simpler treatments currently being used in the literature. Finally, I will outline under what conditions observations of absorption lines from GRBs and Active Galactic Nuclei should be interpreted using our methods, and will give a simple criterion for determining whether a given system must be treated with the full numerical technique or a simpler approximation.