The sharpness of many of the optical absorption and emission lines of the lanthanide ions in ionic crystals has intriguedscientists since 1908. We review some of the recent developments in this area of spectroscopy, emphasizing the optical properties of the tripositive lanthanide and actinide ions.In particular, we shall discuss the single ion properties of line position, intensity, width, and fluorescence lifetime. Such effects as the application of external electric and magnetic fields, hyperfine interactions, and cooperative effects such as long range ordering and energy transfer, although direct extensions of the above properties, must be excluded in such a short review.The optical properties of the lanthanide and actinide ions are due to the unpaired electrons of the ion. The observed sharp transitions have been shown to be intraconfiguration transitions. The most widely studied systems have ground configurations (Xe, 4f n ) and (Rn, 5f n ) for the lanthanide and actinide ions respectively. The number of f-electrons, n, ranges from 1 to 13.The inert rare gas core allows us to discuss the systems in terms of the f-electrons only.Even such a conceptually simple system is complex enough to require a parameterization scheme. The physical significance of such a scheme and its role in developing an understanding of complex systems has been discussed by Newman (1). Our goal is not to uncritically accumulate parameters in some standard scheme which has limited utility, but instead to develop as comprehensive and universal a scheme as possible, one which can be applied to the energy level structure, radiative transition probabilities, temperature-dependent line widths, fluorescent lifetimes, electric and magnetic susceptibilities, hyperfine structure, and cooperative phenomena. In particular, the parameters we deduce should allow us to predict observables in an unmeasured region, be consistent with appropriate ab initio calculations, and be useful as input data into other parameterization schemes. An example of the last point is the analysis 0-8412-0568-X/80/47-131-349$05.00/0