  Understanding the complex behavior of High Mass X-ray binaries (HMXBs) is not
possible without detailed information about their donor stars. While crucial,
this turns out to be a challenge on multiple fronts. First, multi-wavelength
spectroscopy is vital. As such systems can be highly absorbed, this is often
already hard to accomplish. Secondly, even if the spectroscopic data is
available, the determination of reliable stellar parameters requires
sophisticated model atmospheres that accurately describe the outermost layers
and the wind of the donor star.
  For early-type donors, the stellar wind is radiatively driven and there is a
smooth transition between the outermost layers of the star and the wind. The
intricate non-LTE conditions in the winds of hot stars complicate the situation
even further, as proper model atmospheres need to account for a multitude of
physics to accurately provide stellar and wind parameters. The latter are
especially crucial for the so-called "wind-fed" HXMBs, where the captured wind
of the supergiant donor is the only source for the material accreted by the
compact object.
  In this review I will address the different approaches for treating stellar
winds in the analysis of HMXBs. Fundamentals of stellar atmosphere modeling are
discussed, also addressing the limitations of modern models and examples from
recent analysis results for particular HMXBs. Moreover, the path for the next
generation of stellar atmosphere models will be outlined, where models can be
used not only for measurements, but also to make predictions and provide a
laboratory for theoretical conclusions. Stellar atmospheres are a key tool in
understanding HMXBs, e.g. by providing insights about the accretion of stellar
winds onto the compact object, or by placing the studied systems in the correct
evolutionary context in order to identify potential gravitational wave (GW)
progenitors.
