  We present the adaptive optics assisted, near-infrared VLTI instrument -
GRAVITY - for precision narrow-angle astrometry and interferometric phase
referenced imaging of faint objects. Precision astrometry and phase-referenced
interferometric imaging will realize the most advanced vision of
optical/infrared interferometry with the VLT. Our most ambitious science goal
is to study motions within a few times the event horizon size of the Galactic
Center massive black hole and to test General Relativity in its strong field
limit. We define the science reference cases for GRAVITY and derive the top
level requirements for GRAVITY. The installation of the instrument at the VLTI
is planned for 2012.
