<i>Planck</i>pre-launch status: The HFI instrument, from specification to actual performance

<i>Context. <i/>The High Frequency Instrument (HFI) is one of the two focal instruments of the <i>Planck<i/> mission. It will observe the whole sky in six bands in the 100 GHz–1 THz range.<i>Aims. <i/>The HFI instrument is designed to measure the cosmic microwave background (CMB) with a sensitivity limited only by fundamental sources: the photon noise of the CMB itself and the residuals left after the removal of foregrounds. The two high frequency bands will provide full maps of the submillimetre sky, featuring mainly extended and point source foregrounds. Systematic effects must be kept at negligible levels or accurately monitored so that the signal can be corrected. This paper describes the HFI design and its characteristics deduced from ground tests and calibration.<i>Methods. <i/>The HFI instrumental concept and architecture are feasible only by pushing new techniques to their extreme capabilities, mainly: (i) bolometers working at 100 mK and absorbing the radiation in grids; (ii) a dilution cooler providing 100 mK in microgravity conditions; (iii) a new type of AC biased readout electronics and (iv) optical channels using devices inspired from radio and infrared techniques.<i>Results. <i/>The <i>Planck<i/>-HFI instrument performance exceeds requirements for sensitivity and control of systematic effects. During ground-based calibration and tests, it was measured at instrument and system levels to be close to or better than the goal specification.