X-ray Diagnostics of Astrophysical Plasmas John Raymond Harvard-Smithsonian Center for Astrophysics 60 Garden Street Cambridge, MA 02138 This is a brief historical overview of X-ray diagnostics and of the general codes that predict thermal X-ray emission spectra. It will discuss the general approaches behind different X-ray models and the general ideas behind line ratio diagnostics for density, temperature and optical depth. It will also discuss some of the things that might be added to the codes and the level of accuracy available.