An OH/IR star is an intermediate mass AGB star which loses mass and is optically obscured by its envelope. Such stars can be detected in the infrared and by radio maser lines. In particular the OH line (1612 MHz) is bright and has a distinctive profile, that allows us to determine the radial velocity of the central star. OH/IR stars are excellent tracers for the study of Galactic dynamics because they are bright, unaffected by extinction in the OH line – and therefore visible throughout the Galaxy – and because they represent a wide range of stellar masses. The outflow velocity of the circumstellar shell is most likely driven by radiation pressure from the central star on the dust. Therefore outflow velocity increases with stellar luminosity and metallicity, and decreases with age.