The role of atomic physics in understanding physical processes in high energy astrophysics

X-ray grating spectra from Chandra and XMM-Newton have provided new insights into many of the physical processes present in astrophysical sources. For example, (i) shocks produced by magnetic accretion onto stellar surfaces cool as the material flows down, with density and temperature diagnostics providing tests of the accretion models; (ii) many active galactic nuclei (AGN) produce winds or outflows, detectable through X-ray absorption; (iii) active cool stars have coronal pressures several orders of magnitude larger than found on the Sun. The diagnostics used to determine temperatures, densities, elemental abundances, ionization states, and opacities require extremely accurate atomic data. At the same time, we must have a fairly complete database in order to ensure that the diagnostics are not blended or otherwise compromised. The best spectra are from bright objects with long exposures (days), but the information contained allows us to infer the location(s) of the emitting and absorbing plasmas and understand the physical properties. We will give examples to illustrate the role of atomic physics in our analyses of such spectra and the quality of data required.