The diagnostics functions of neutron measurements are reviewed as well as the roles played by neutron yield monitors, cameras and spectrometers. The importance of recent developments in Neutron Emission Spectroscopy (NES) diagnostics is emphasized. Results are presented from NES diagnosis of Joint European Torus (JET) plasmas performed with the Magnetic Proton Recoil (MPR) spectrometer during the first Deuterium Tritium Experiment (DTE1) of 1997 and the recent Trace Tritium Experiment (TTE) of 2003. The NES diagnostic capabilities at JET are presently being enhanced by an upgrade of the MPR (MPRu) and a new 2.5-MeV Time-Of-Flight (TOF) neutron spectrometer (TOFOR). The principles of MPRu and TOFOR are described and illustrated with the diagnostic role they will play in the high performance fusion experiments in the forward program of JET largely aimed at supporting the International Thermonuclear Experimental Reactor (ITER). The importance for the JET NES effort for ITER is discussed.