  Stars between about 4 and 25 solar masses spend a significant fraction of
their post-main sequence lifetime as red supergiants (RSGs) and lose material
via stellar winds during this period. For RSGs more massive than 10 solar
masses, this mass loss becomes of evolutionary significance, and probably
determines the upper mass limit of RSGs in the Hertzsprung-Russell diagram.
Despite decades of observations, the driving mechanism responsible for mass
loss in RSGs remains unknown. Mainly this is because the optical spectrum
accessible from the ground provides almost no useful wind diagnostics, and what
information is obtained is spatially averaged over the entire wind volume.
However, within the last decade, Hubble Space Telescope (HST) observations of
many useful ultraviolet wind diagnostics have been obtained at a high
signal-to-noise ratio and spectral resolution. In particular, RSGs in eclipsing
binaries can provide spatially resolved observations of stellar chromospheres
and winds. I review possible RSG wind acceleration mechanisms, discuss some
observational constraints, and present some empirical models of RSG
chromospheres and winds.
