The status of active beam spectroscopy at the JET tokamak is described and an overview is given of the main developments of charge exchange (CXRS) and beam emission spectroscopy (BES). The recent preliminary tritium experiment at JET, where CXRS has provided ion temperature and deuteron density profiles, and BES has given an account of the tritium beam Balmer Alpha emission, has been a test case for quantitative spectroscopy. Magnetic field studies based on the motional Stark effect have led to precise measurements of the toroidal field and its local changes caused by diamagnetic effects. Absolute measurements of alpha particle densities and reconstruction of anisotropic slowing-down distribution functions are demonstrated for the case of helium neutral beam injection. Feasibility studies for alpha particle detection by CXRS are extrapolated to future fusion devices. The progress in the quality of atomic data is also illustrated by successful modelling of cross-section effects on observed line shapes which allows the prediction of apparent Doppler shifts, Doppler widths and ·line intensities.