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GRAVITY is a second generation instrument for the VLT Interferometer,designed to enhance the near-infrared astrometric and spectro-imagingcapabilities of VLTI. Combining beams from four telescopes, GRAVITY willprovide an astrometric precision of order 10 micro-arcseconds, imagingresolution of 4 milli-arcseconds, and low and medium resolutionspectro-interferometry, pushing its performance far beyond current infraredinterfero- metric capabilities. To maximise the performance of GRAVITY,adaptive optics correction will be implemented at each of the VLT UnitTelescopes to correct for the effects of atmospheric turbulence. To achievethis, the GRAVITY project includes a development programme for four newwavefront sensors (WFS) and NIR-optimized real time control system. Thesedevices will enable closed-loop adaptive correction at the four Unit Telescopesin the range 1.4-2.4 {\mu}m. This is crucially important for an efficientadaptive optics implementation in regions where optically bright referencessources are scarce, such as the Galactic Centre. We present here the design ofthe GRAVITY wavefront sensors and give an overview of the expected adaptiveoptics performance under typical observing conditions. Benefiting from newlydeveloped SELEX/ESO SAPHIRA electron avalanche photodiode (eAPD) detectorsproviding fast readout with low noise in the near-infrared, the AO systems areexpected to achieve residual wavefront errors of \leq400 nm at an operatingfrequency of 500 Hz.

The GRAVITY Coud\'e Infrared Adaptive Optics (CIAO) system for the VLT Interferometer