SIM PlanetQuest is a very high accuracy space astrometric instrument based on a long baseline stellar interferometer. For global astrometry SIM was designed to be accurate to ≈ 4μas (microarcsec) after a 5 year mission. For narrow angle astrometry (≈ 1000 s integration over a 1° radius field) SIM is designed for 1μas precision. The technology program was completed in 2005 and based on laboratory results, the current best estimate of SIM's performance would be 0.6 μas for narrow angle precision and 2.4 μas for global accuracy. This paper describes a variety of science programs that the SIM science team have proposed to conduct from a search for one Earth mass planets in the habitable zone of (≈130) nearby stars to the study of dark matter in the galactic disk, the galactic halo and the local group.