  Observations with powerful X-ray telescopes, such as XMM-Newton and Chandra,
significantly advance our understanding of massive stars. Nearly all early-type
stars are X-ray sources. Studies of their X-ray emission provide important
diagnostics of stellar winds. High-resolution X-ray spectra of O-type stars are
well explained when stellar wind clumping is taking into account, providing
further support to a modern picture of stellar winds as non-stationary,
inhomogeneous outflows. X-ray variability is detected from such winds, on time
scales likely associated with stellar rotation. High-resolution X-ray
spectroscopy indicates that the winds of late O-type stars are predominantly in
a hot phase. Consequently, X-rays provide the best observational window to
study these winds. X-ray spectroscopy of evolved, Wolf-Rayet type, stars allows
to probe their powerful metal enhanced winds, while the mechanisms responsible
for the X-ray emission of these stars are not yet understood.
