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.