Given high resolution X-ray spectra, one can derive the temperature structure, elemental abundances, density, and in some cases heating or cooling rate of a hot astrophysical plasma. The accuracy of various atomic rate coefficients limits the accuracy of the inferred plasma parameters. The important rates are typically collisional ionization, radiative and dielectronic recombination, collisional excitation, and radiative decay. Of these, dielectronic recombination is often among the most uncertain. Accurate collisional excitation rates are available for strong transitions of simple ions, but a disturbing number of excitation rates are only known to a factor of two.