1 Introduction

The 0.2 to 10 keV energy band contains the K-shell transitions of carbon, nitrogen, oxygen, neon, magnesium, silicon, sulphur and calcium, as well as both the L and K shell transitions of iron. Of particular interest, due to their high abundances, are oxygen and iron at temperatures of 10⁶ K and 10⁷-10⁸ K, respectively. Detailed analysis of spectral features from these elements will permit the determination of density, temperature, ionisation state, elemental abundances, mass motions and velocity shifts of the emitting regions and their surrounding environments. The spectral properties of the medium will depend on the excitation process (collisional or photo-ionization and recombination), the state (transient or quiescent) and on whether the plasma is optically thick or thin. With RGS it is possible to measure most of these transitions with the notable exception of those associated with the Fe K-shell. Amongst others, observations with RGS will give new information about the heating of stellar coronae, the propagation of supernova remnant shocks through the interstellar medium, the conditions in the circumsource environments of X-ray binaries and active galactic nuclei, and the hot intracluster medium of clusters of galaxies.