Fe XVIII produces, in the X-ray and extreme ultraviolet, L-shell (n=2,3,4->2) spectral lines which are among the brightest ones in e.g. solar flares and in Chandra, XMM-Newton spectra of active stars. Recent R-matrix scattering calculations of Witthoeft et al. (2006A&A...446..361W) produce theoretical intensities for some of the brightest transitions increased by large factors (2-3), so it is timely to use these calculations to review and assess all previous line identifications on a quantitative basis. This paper discusses only the most important lines for laboratory and astrophysical applications. Many previous identifications are revised and some tentative ones finally confirmed. Many lines are found to be significantly blended. A considerable number of new identifications are proposed. Excellent agreement between observed and predicted intensities is found in the majority of cases for the first time. It is therefore now possible to use Fe XVIII L-shell lines to measure electron densities in laboratory plasmas and temperatures for a wide range of astrophysical sources.