Astrophysical X-ray spectroscopy promises huge potential scientific returns. The soft X-ray bandpass, 0.1-10 keV, contains transitions from the K-, L-, and M-shell of every cosmically abundant element and ion except H and He. With only moderate (R ~ 1000) resolution, these transitions can be separated into gas, molecular, and solid state phases. Line and continuum measurements at lower resolutions (R ~ 100) can determine the electron temperature, estimate the electron density or radiation field and reveal if the plasma is in equilibrium. Achieving these returns, however, requires accurate data for the underlying rates and transition wavelengths for ions, molecules and solid state materials. Uncertainties in the oscillator strengths of Fe XVII transitions already limit the conclusions that can be made about the non-thermal turbulence in two galaxy groups (de Plaa et al., 2012), while the paucity of accurate wavelengths and collisional rates in the 50-150 Å bandpass have affected analysis of data from the Chandra X-ray Observatory's Low-Energy Transmission Grating (LETG) (e.g., and ). We describe the atomic physics required for the X-ray diagnostics that are in use with existing X-ray missions and that will be required for future X-ray missions.