Astrometric measurement techniques

This contribution reviews the various principles which form the basis of astrometric measurements, such as meridian circle observations, imaging techniques, scanning satellites and interferometry. Various projects which employ those techniques are described, e.g. the Multichannel Astrometric Photometer, the Fine Guidance Sensors onboard the Hubble Space Telescope, the Hipparcos, Gaia and SIM Lite missions as well as the PRIMA instrument. This review covers both relative as well as absolute astrometric measurement techniques, from several milliarcsecond to microarcsecond precision.