A cryo-condensation pump has been installed inside the JET vacuum vessel as part of the divertor programme. The stringent requirements of the JET operational cycle and the hostile tokamak environment, presented particular challenges to the engineering of the pump. The design has to cope with high eddy current forces from the plasma operation and be compatible with in-situ baking to 573K prior to subsequent operation at 77K. The pump also has to fulfil stringent requirements relating to overall reliability and operational safety in a variety of possible fault scenarios (loss of torus vacuum, cooling water, cryogens, etc.). The design, manufacture, assembly, quality control, testing and operational safety of the pump are described. Unstable flow regimes in the cryogenic loops are prevented by circulating the cryogens in pipework which is connected to give a series flow arrangement for both liquid nitrogen (LN) and supercritical helium (LHe) circuits. Pumping of hydrogen and helium gas will be performed using cryosorption onto argon which will be pre-condensed onto the helium cooled surfaces. With regard to operational safety, a detailed heat transfer model which incorporates radiative, conductive and convective heat transfer has been developed to predict the time-dependent temperature differences in the liquid nitrogen (LN) cooled structure of the cryopump. In addition, the behaviour of surrounding components for a wide range of adverse scenarios has also been investigated. Analysis of the behaviour of water cooled components in the vicinity of the cryopump which are at risk of freezing due to the proximity of the LN cooled structure of the cryopump, has resulted in establishing criteria for the water flow and vacuum vessel temperature required to ensure safe operation for an extensive range of envisaged scenarios.