Open Issues Concerning Ionization J. Martin Laming Naval Research Laboratory Code 7674L Washington DC 20375 I will survey the state of atomic data relevant to calculations of ionization balance, and highlight applications of plasma diagnostics where uncertainties in ionization balance may play an important role. Such effects, and strategies for dealing with them are by now well known in the case of plasmas in collisional ionization equilibrium, such as stellar coronae. However time-dependent plasmas, either ionizing or recombining, where the departure from ionization equilibrium is itself an important diagnostic, or static plasma where radiative instability plays a role, place new demands on the accuracy of the atomic data. I will illustrate such issues with examples from x-ray observations of supernova remnants and modeling work on winds from accreting black holes.