Theoretical work and observations on extraterrestrial Lyman α are reviewed. A source of extraterrestrial Lyman α near the solar apex is probably scattering of solar Lyman α on cold interplanetary hydrogen that has penetrated to the inner solar system. A minor source of interplanetary hydrogen comes from the breakup of comets. Galactic sources of Lyman α are probably also present, and such emission would tend to be rendered isotropic, depending on the amount of interstellar hydrogen in the near vicinity of the solar sys