Abstract
Ultraviolet emission line ratios have long provided powerful diagnostics for the density, temperature, elemental abundances and ionization state of astrophysical plasmas. With the current generation of X-ray satellites, these techniques can be applied to X-ray spectra. It is important to remember that any such line ratio provides an average value along the line of sight, and different diagnostics will provide differently weighted averages. Several assumptions are often made implicitly when emission line ratios are analysed. In particular, the optical depth is generally assumed to be negligible. This paper considers some examples of the opportunities provided by photon scattering for new diagnostic tools, and it considers the dangers of applying the standard methods when the optical depth is significant.