  Massive stars, at least $\sim$ 10 times more massive than the Sun, have two
key properties that make them the main drivers of evolution of star clusters,
galaxies, and the Universe as a whole. On the one hand, the outer layers of
massive stars are so hot that they produce most of the ionizing ultraviolet
radiation of galaxies; in fact, the first massive stars helped to re-ionize the
Universe after its Dark Ages. Another important property of massive stars are
the strong stellar winds and outflows they produce. This mass loss, and finally
the explosion of a massive star as a supernova or a gamma-ray burst, provide a
significant input of mechanical and radiative energy into the interstellar
space. These two properties together make massive stars one of the most
important cosmic engines: they trigger the star formation and enrich the
interstellar medium with heavy elements, that ultimately leads to formation of
Earth-like rocky planets and the development of complex life. The study of
massive star winds is thus a truly multidisciplinary field and has a wide
impact on different areas of astronomy. [...]
  This detailed review summarises the current knowledge on the theory and
observations of winds from massive stars, as well as on observations and
accretion processes in wind-fed high mass X-ray binaries. The aim is to combine
in the near future all available theoretical diagnostics and observational
measurements to achieve a unified picture of massive star winds in isolated
objects and in binary systems.
