The surfaces of interstellar and interplanetary grains, planetary satellites, ring particles, asteroids, Mercury and the Moon, etc. are all directly exposed to radiations. Solar wind, magnetospheric plasmas, cosmic rays, solar energetic particles, and U.V. photons can all produce chemical and physical alterations. These alterations affect the surface reflection properties and can eject species into the gas phase affecting the ambient gas and plasma. The physical processes involved are reviewed and the relevant data discussed. These are then applied to a number of outer solar system problems.