<p>The discovery of exoplanets has opened a new, very rich field in modern astronomy, allowing a much better insight into the characteristics of planets and the mechanisms for their formation than possible from only the observation of the Solar System. After a short resume of the methods used to observe exoplanets, we present the various mechanisms considered for their formation, with a focus on the most massive and easy to observe planets, the gas giants similar to or more massive than Jupiter. Their formation should be considered in the general framework of the formation of stars and their companions. Recent observations clearly show that companions of stars at separation less than a few hundred au mainly divide into two groups: massive ones, including stellar and massive brown dwarf companions, and low-mass ones, including planets and low-mass brown dwarfs, separated by the brown dwarf desert. The first group likely formed by gravitational instability in the protostellar disc, while the second one likely formed by the core accretion mechanism. The frequency of planets like or more massive than Jupiter is found to depend on the environment, being high in the smallest star-forming regions and much lower in the largest ones. This is discussed in relation to the formation of the Solar System.</p>