This paper reports recent progress in the classification of the spectra of iron XVIII to XXIV. Emission lines due to these stages of ionisation were reported in the spectra of solar flares by Neupert et al. (1967) who pointed out that the 2s2 2pn–2s2 2pn-1 3d configurations were mainly responsible. The present work verifies this and in addition gives individual line classifications of some of the strongest lines. Earlier studies of these transitions were reported by Fawcett (1965), Fawcett et al. (1967), and Feldman and Cohen (1968, 1970). Spectra of Fe XVIII to XXIV dueto 2s2 2pn–2s 2pn+1 transitions lie near 90 Å. Satisfactory solar and laboratory observations of these spectra are not yet available. The new classifications in these isoelectronic sequences are extended almost as far as iron and enable accurate extrapolations to iron. Isoelectronic spectra of both the forementioned transitions have been studied and the wavelengths tabulated (Fawcett, 1970, 1971a, b; Fawcett et al., 1971) for all the elements in the period between silicon and iron. The wavelength data enables the calculation of the 2s2 2pn energy levels and hence provides confirmation of the identity of solar forbidden lines in silicon, sulphur, argon and calcium. These energy levels are in close agreement with the theoretical calculations of Edlén reported at this Conference.