X-ray radiation damage to cryocooled ( approximately 100 K) macromolecular crystals has emerged as a general problem, especially since the advent of third generation synchrotron undulator sources. Interest in understanding the physical and chemical phenomena behind the observed effects is growing rapidly. The specific structural damage seen in electron density maps has to be accounted for when studying intermediates, and can sometimes be related to biological function. Radiation damage induces non-isomorphism, thus hampering traditional phasing methods. However, specific damage can also be used to obtain phases. With an increased knowledge of expected crystal lifetime, beamline characteristics and types of damage, macromolecular crystallographers might soon be able to account for radiation damage in data collection, processing and phasing.