HARPS is a high-resolution spectrograph designed to achieve long-term stability enabling Doppler velocity measurements at the level of 1 m/s, in order to derive radial-velocity orbits for stars orbited by planets as small as the Earth. It is presently the only instrument scheduled on the 3.6-m telescope at the European Southern Observatory on La Silla in Chile. The spectrograph is fed by fibers and the optics are enclosed in a vacuum tank, to eliminate the effects of changes in the index of refraction at the echelle grating due to changes in the atmospheric pressure. The temperature of the instrument is stabilized at the level of 0.001 K, which is also critical for the velocity stability. The wavelength calibration is provided by thorium-argon hollow cathode lamps, but experiments are underway with the use of Fabry-Perot etalons and/or Laser Combs as calibration sources. HARPS has demonstrated the potential to reach a...