Although substantial progress has been achieved since the discovery of X-ray emission from early-type stars with the EINSTEIN satellite, several crucial aspects of this phenomenon are still not fully understood. Considerable breakthroughs in this field are expected from observations with the X-ray Multi-Mirror satellite (XMM) due for launch in early 2000. XMM is the second cornerstone mission of the ESA Horizon 2000 science programme (see Lumb et al. 1996 and references therein for an overall description of the satellite). XMM offers a large effective area over a wide range of energies and its instrumentation provides simultaneously non-dispersive spectroscopic imaging (EPIC - European Photon Imaging Camera), medium-resolution dispersive spectroscopy (RGS - Reflection Grating Spectrometer) and optical-UV imaging (OM - Optical Monitor).