SIM PlanetQuest is a planned orbiting space interferometer that uses microarcsecond astrometry to detect extrasolar terrestrial planets and measure parallaxes and proper motions across the galaxy. Development of the required technologies, including picometer precision laser metrology, nanometer precision pathlength control, novel optical elements, broadband beam combination, and accurate optical modeling techniques, have evolved from components, through subsystems, and finally to system-level testbeds that demonstrate the full complexity and required performance of the instrument. SIM Planetquest is now in the engineering phase and is poised to begin implementation for a launch within 5 years.