  Nearly all types of massive stars with radiatively driven stellar winds are
X-ray sources that can be observed by the presently operating powerful X-ray
telescopes. In this review I briefly address recent advances in our
understanding of stellar winds obtained from X-ray observations. The winds of
OB dwarfs with subtypes later than O9V may be predominantly in a hot phase, and
X-ray observations offer the best window for their studies. The X-ray
properties of OB supergiants are largely determined by the effects of radiative
transfer in their clumped stellar winds. The recently suggested method to
directly measure mass-loss rates of O stars by fitting the shapes of X-ray
emission lines is considered but its validity cannot be confirmed. To obtain
robust quantitative information on stellar wind parameters from X-ray
spectroscopy, a multiwavelength analysis by means of stellar atmosphere models
is required. Independent groups are now performing such analyses with
encouraging results. Joint analyses of optical, UV, and X-ray spectra of OB
supergiants yield consistent mass-loss rates. Depending on the adopted clumping
parameters, the empirically derived mass-loss rates are comparable (within a
factor of a few) to those predicted by standard recipes (Vink et al. 2001). All
sufficiently studied O stars display variable X-ray emission that might be
related to corotating interaction regions in their winds. In the latest stages
of stellar evolution, single red supergiants and luminous blue variable stars
do not emit observable amounts of X-rays. On the other hand, nearly all types
of Wolf-Rayet (WR) stars are X-ray sources. X-ray spectroscopy allows a
sensitive probe of WR wind abundances and opacities.
