In recent years, the study of the mechanisms of formation and evolution of planetary systems has received a considerable boost from the discovery of more than 260 extra-solar planets, mainly thanks to the analysis of the variations of radial velocities of the stars. While several general features of planetary systems are beginning to emerge, still little is known of several aspects, concerning for example the possible mechanisms that lead to the observed planet configurations (semi major axis, orbital eccentricity, planetary masses, etc.). This has significant impact on the determination of the frequency of planets in general, and of those able to host life in particular.

The structure of the paper is as follows. In part 1 an historic presentation of the topic is given.

In part 2 the methods utilized for the detection of the exoplanets are described. A special emphasis on the radial velocity technique, astrometry and photometry is given and a general review on other methods related to gravitational microlensing, pulsar timing, direct detention, etc is offered.

In part 3, some general aspects about ongoing research into extra solar planets are examined.
Some ground programmes such as PLANET, HARPS, ELODIE are shown and some ongoing space missions, e.g. COROT (CNES-ESA), and Future Projects such as KEPLER (NASA) and DARWIN (ESA) are presented.

In part 4, current theories about planet formation, like the core accretion and the disk instability, as well as proposed mechanisms of planet migration to explain the presence of massive planets in very close orbits around their host stars, are briefly presented and commented on. Than, general properties of the exoplanets discovered and characteristics of notable extra solar planets are discussed. Moreover, the singularities of the Habitable Zone and the spectrum indicators of the biological activity are shown.