There are many systems which present long range interactions, such gravitational systems, two-dimensional vortices, cold atoms, etc. In this talk, I will start explaining the scenario of their evolution, which consists first in the generic formation of out-of-equilibrium long lived states, called "Quasi Stationary States" and then a (comparatively) very slow relaxation towards thermal equilibrium. Then, I will focus on the latter mechanism, showing recent theoretical results and numerical simulations on the collisional relaxation mechanism.