In planetary systems populated by two or more giant planets, planet--planet scattering can lead to collisions and ejections of one or more bodies. The original planetary configuration can be significantly altered by this kind of dynamical evolution and the presently observed system would be very different from the original outcome of the planet formation process. For scattering to occur, either the planets formed in an unstable configuration or migrated into it because of tidal interactions with the protostellar disk. The strong gravitational interactions typical of the chaotic phase of planet--planet scattering cause large changes in the orbital elements of the planetary bodies. The distribution of orbital eccentricities for the known extrasolar planets, with values even exceeding 0.9, may easily arise through planet-planet interactions. In addition, the combination of planet--planet scattering and tidal circularization can explain some of the orbits of the so called ''Hot Jupiters'', giant planets with very short period orbits. In this context, the efficiency of close--in planets production is significantly increased by Kozai oscillations induced by the outer planet(s) on the inner one. The chaotic evolution of the system dominated by close encounters between the planets depends on the presence of a residual gas disk which may affect the interactions between the planetary bodies. In binary star systems this evolution is even more complex and the gravitational perturbations of the companion star may influence the outcome of the planet--planet scattering phase.