Heavy ion cooler rings have been installed in a number of laboratories around the world. They allow for a new generation of experiments using the products of high energy heavy ion collisions, i.e. highly charged and possibly radioactive ions. The first series of investigations at the SIS-ESR facility at the GSI, reviewed here, includes the observation of beta decay into a bound state of a highly charged ion and precision spectroscopy of inner-shell levels by means of X-ray spectroscopy, laser spectroscopy and resonant di-electronic recombination.