The JET tokamak relies on an active charge exchange spectroscopy diagnostic for measurements of ion temperature, toroidal rotation velocity and impurity density profiles. It uses the neutral heating beams as diagnostic beams and provides measurements at 8 to 12 radial positions simultaneously with a time resolution of about 100ms. A description of the instrument is given, together with a presentation on the analysis of the experimental data and an account on recent results. Profiles of ion temperature and rotation frequency based on the carbon CVI n = B to n = 7 transition at 5290.5Å are presented for plasmas in the hot ion mode, for magnetic limiter plasmas, and for discharges where a locked MHD mode brings the plasma to rest despite continued neutral beam injection. H to L-mode transitions are found to cause a collapse of the edge ion toroidal rotation frequency within less than a sampling time time resolution is also sufficient to reveal ion heat sawtooth crashes.