X-ray astronomy primarily involves the study of plasmas with thermal temperatures in the range of 1E6 to 1E8 K. Such plasmas radiate the bulk of their energy in the X-ray regime, from ca. 0.1 to 10 keV. ... Apart from X-ray continuum emission, produced through processes such as e.g. thermal bremsstrahlung, a significant fraction of the total emissivity of hot thermal plasmas can arise from line emission. ... XMM's science objectives are described here in two parts: . General science objectives . ...