HIRES (High Resolution Echelle Spectrometer) is a high-resolution spectrograph on the Keck I telescope in Hawaii. It was originally designed as a general-purpose instrument, but one of its main applications has been the determination of radial-velocity orbits for stars hosting exoplanets in order to derive planetary masses. Because the input is a slit, an iodine gas absorption cell is introduced into the stellar beam in order to calibrate the instrumental profile and wavelength scale of each stellar observation. The data analysis requires also a high-quality template observation of each star without the iodine and a fundamental calibration of the spectrum of the absorption cell at very high spectral resolution. HIRES has achieved a performance approaching velocities as low as 1 m/s. HIRES has contributed to the discovery of several hundred extrasolar planets including the first super-Earth, GJ 876 d, several potentially habitable...