Dielectronic recombination is a process particularly effective in high-temperature dilute plasmas such as those observed, for example, in astrophysics. This process was first recognised for its noticeable effects on the establishment of ionisation equilibrium in the solar corona. Over the last decade, its importance as a line formation mechanism has been emphasised. It is responsible for spectral lines appearing as satellites to the long-wavelength side of the resonance lines of the highly ionised systems typical of hot plasmas. However, in transient ionising plasmas and for the heavier ions these lines can also be produced by direct excitation of an innershell electron. This review is concerned essentially with the analysis and interpretation of the satellite lines in terms of the physical parameters characterising the emitting plasma. The theory for the satellite intensities is worked out in detail and the calculation methods of the related atomic parameters (such as wavelengths, transition probabilities, autoionisation rates, etc) are described. Extensive results concerning the astrophysically important hydrogen-like and helium-like ions are summarised. These are compared with the few available high-resolution soft x-ray spectra of solar active regions and flares and show good agreement mainly in the stronger lines. Such comparisons allow both the electron temperature and the transient state of the plasma to be determined within limitations connected mainly with the quality of the present observations.

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