Plasma with temperatures above a million Kelvin is common in the Universe a
nd exists for a large part in the intergalactic space, but is also found in
a variety of discrete cosmic sources that range from optically thin to thi
ck. I consider briefly the general outline of X-ray spectral modelling of h
ot plasmas, in particular the optically thin model applicable to sources li
ke stellar coronae, clusters of galaxies, and supernova remnants. I deal wi
th the diagnostics of plasma parameters to be obtained from optically thin
line and continuum X-ray spectra and give some examples of spectral simulat
ions for various spectrometers on board Spectrum-XG.