Overview and working principle of GRAVITY: the instrument coherently combines the light of the four 8 m UTs of the VLTI or the four 1.8 m ATs. It provides infrared wavefront sensing to control the telescope adaptive optics, two interferometric beam combiners − one for fringe-tracking and one for the science object −, an acquisition camera and various laser guiding systems for beam stabilization, and a dedicated laser metrology to trace the optical path length differences for narrow angle astrometry. The overview illustrates the light path and location of the various subsystems. For clarity, we show only two telescopes, one beam combiner, and one wavefront sensor. The wavefront sensor star (green) can be outside the VLTI field of view. Depending on the brightness of the science object (red), the fringe tracker is either fed by a beam splitter, or − as illustrated in this figure − by a bright off-axis phase reference star (blue). The GRAVITY subsystems (light blue boxes) are embedded and take advantage of the already existing VLTI infrastructure (light red boxes).