  Over 300 extrasolar planets have been found since 1992, showing that
planetary systems are common and exhibit an outstanding variety of
characteristics. As the number of detections grows and as models of planet
formation progress to account for the existence of these new worlds,
statistical studies and confrontations of observation with theory allow to
progressively unravel the key processes underlying planet formation. In this
chapter we review the dominant contribution of Doppler spectroscopy to the
present discoveries and to our general understanding of planetary systems. We
also emphasize the synergy of Doppler spectroscopy and transit photometry in
characterizing the physical properties of transiting extrasolar planets. As we
will see, Doppler spectroscopy has not reached its limits yet and it will
undoubtly play a leading role in the detection and characterization of the
first Earth-mass planets.
