[en] Soft X-ray spectra with high resolution represent a rich source of information on the nature of solar plasma as well as on the power loss of the fusion plasma due to impurity radiation. These emission spectra correspond to multiply ionized atoms and for their interpretation in terms of physical parameters it is necessary to understand the atomic processes involved and to compute the corresponding parameters. These X-ray diagnostic techniques are used mainly for active region and flare plasmas. Spectra from calcium and iron have been observed and fitted with the best available theoretical spectra. (Auth.)