  We present the proposal for the infrared adaptive optics (AO) assisted,
two-object, high-throughput, multiple-beam-combiner GRAVITY for the VLTI. This
instrument will be optimized for phase-referenced interferometric imaging and
narrow-angle astrometry of faint, red objects. Following the scientific
drivers, we analyze the VLTI infrastructure, and subsequently derive the
requirements and concept for the optimum instrument. The analysis can be
summarized with the need for highest sensitivity, phase referenced imaging and
astrometry of two objects in the VLTI beam, and infrared wavefront-sensing.
Consequently our proposed instrument allows the observations of faint, red
objects with its internal infrared wavefront sensor, pushes the optical
throughput by restricting observations to K-band at low and medium spectral
resolution, and is fully enclosed in a cryostat for optimum background
suppression and stability. Our instrument will thus increase the sensitivity of
the VLTI significantly beyond the present capabilities. With its two fibers per
telescope beam, GRAVITY will not only allow the simultaneous observations of
two objects, but will also push the astrometric accuracy for UTs to 10
micro-arcsec, and provide simultaneous astrometry for up to six baselines.
