  Charge-exchange reactions between outflowing stellar wind protons and
interstellar neutral hydrogen atoms entering a stellar astrosphere produce a
region of piled-up-decelerated neutral hydrogen called the hydrogen wall.
Absorption by this gas, which is observed in stellar Lyman-alpha emission
lines, provides the only viable technique at this time for measuring the
mass-loss rates of F-M dwarf stars. We describe this technique, present an
alternative way for understanding the relation of mass-loss rate with X-ray
emission, and identify several critical issues.
