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ITER operation relies on heating neutral beam injectors (HNB) based on negative ion deuterium beams accelerated at 1 MeV, with up to one hour 17 MW power, uniform intensity and low divergence. A neutral beam test facility is being built at Consorzio RFX to demonstrate the feasibility of a prototype injector with such demanding specifications and to optimize its performances. It comprises SPIDER, a 100 kV negative hydrogen/deuterium RF source, full size prototype of the HNB source, and MITICA, a prototype of the full HNB. The diagnostics for SPIDER and MITICA are essential to qualify and optimize the HNB for ITER and to assess the information on source and beam obtainable with the reduced set of the HNB diagnostics. Main parameters are measured with different complementary techniques to exploit the combination of their specific features. In the source the still open issues of cesium dynamics, negative ion generation and uniformity are investigated with a set of electrostatic probes, with optical emission spectroscopy, laser absorption for neutral cesium density and cavity ring down spectroscopy for negative ion density. The beam profile uniformity and its divergence will be studied with beam emission spectroscopy, visible tomography, neutron imaging and a special calorimeter made of carbon tiles which can achieve a spatial resolution of few millimetres. All components heated by beam power load will be monitored with a large set of thermocouples behind the heated surfaces and on the cooling water circuits. The test facility will also be the natural choice for hosting experiments on innovative concepts for DEMO HNBs, e.g. more efficient neutralization techniques and cesium free source operation.