This paper describes the Heterodyne Instrument for the Far-Infrared (HIFI) to be launched onboard of ESA's Herschel Space Observatory, by 2008. The instrument is designed to be electronically tunable over a wide and continuous frequency range in the Far Infrared, with velocity resolutions better than 0.1 km s-1 and a high sensitivity. This will allow detailed investigations of a wide variety of astronomical sources, ranging from solar system objects, star formation regions to nuclei of galaxies. The instrument comprises 5 frequency bands covering 480–1150 GHz with SIS mixers and a sixth dual frequency band, for the 1410–1910 GHz range, with Hot Electron Bolometer Mixers (HEB). The Local Oscillator (LO) subsystem consists of a Ka-band synthesizer followed by 14 chains of frequency multipliers, 2 chains for each frequency band. A pair of Auto-Correlators and a pair of Acousto-Optic spectrometers process the two IF signals from the dual-polarization front-ends to provide instantaneous frequency coverage of 4 GHz, with a set of resolutions (140 kHz to 1 MHz), better than <0.1 km s-1. After a successful qualification program, the flight instrument entered the testing phase. We will also report on the first pre-flight test and calibration results together with the expected in-flight performance.