The fast ion and alpha-particle diagnostic is based on collective Thomson scattering of high power 140 GHz radiation.The main aim of the system will be the determination of the spatially resolved velocity distribution of fast ion populationsin JET plasmas, by measuring the frequency spectrum of scattered radiation. A spatial profile of the velocity distribution,with resolution <10 cm, can be determined by scanning the scattering volume (the overlap of the antenna patterns of thelaunch and receive antennas) over the plasma, using steerable launch and receive mirrors. The diagnostic will be used tomeasure fast ion populations produced by the heating systems, and will be particularly important in the tritium phase ofJET when, by a measurement of the velocity distribution of alpha particles created by fusion reactions, the efficiency ofalpha particle heating can be established. It is expected that their velocity distribution can be determined between 0.5MeV (limited because at low frequency shift the spectrum is dominated by the thermal ion feature) and their birth energy(3.5 MeV) with a typical time resolution of 0.1 s. The principal components of the diagnostic are shown schematically infigure 1. They are: a high power long pulse gyrotron and associated equipment, a heterodyne receiver system andtransmission lines. The RF transmission and the receiver system are described in accompanying papers1,2. The physicsprinciples of the diagnostic are described elsewheree.g. 3'4.