Shocks occur in many astrophysical environments. Most of the shocks are collisionless, mediated by collective plasma effects. However, shocks that propagate in very dense environments will be directly mediated by radiation, with completely different microphysics as a result. For instance, these shocks are unable to accelerate charged particles. Radiation mediated shocks (RMSs) are found in all sorts of stellar explosions, such as supernovae and gamma-ray bursts. I will give an introduction to RMSs, contrasting them to collisionless shocks, and describe the latest developments in the field. I will finish by discussing currently unsolved problems related to RMSs, and how they could be related to the gamma-rays that were observed from last years neutron star merger event.