Atomic reaction models provide the link by which quantitative diagnostic comments on plasma behaviour and parameters may be made from spectral observations of emission by ions in the plasma. The paper reviews progress in this area with emphasis on impurity species in magnetic confinement fusion plasmas. A systematic approach based on generalised collisional-radiative theory is adopted and items discussed include spectroscopy of edge and divertor environments, beam emission and charge exchange spectroscopy. Case studies are based on experience at the JET Joint European Torus Experiment.