[en] A magnetic proton recoil (MPR) spectrometer has been designed and constructed for use at the Joint European Torus (JET) tokamak. It was installed at JET in October 1996 and has subsequently been in continuous use during the first main deuterium-tritium (DT) operation in 1997, which culminated in the production of a record plasma generating 16.1 MW fusion power. This thesis describes the MPR spectrometer, including calibration and characterization procedures, and the JET experiments. Special emphasis is given to the focal plane detector and its response to radiation, which is a part of the total spectrometer response function. The result from this development and characterization work is presented. The data taking and reduction procedures are also described. Results from the first measurements in D-plasmas on the triton burnup neutron energy distribution are presented and compared with calculations. Examples of results obtained for DT-plasmas are reported which represent the first use of neutron spectrometry for measurements of plasma rotation and fast ion components in radio frequency heated plasmas. The forthcoming use of neutron spectrometry, especially for studies of ignited plasmas, is outlined in view of gained operating experience of the new technique and the achieved results