We review the elementary theory of elastic binary collisions and the statistical concepts that can be derived from it (a very simple introduction to this concepts can be found on [98]). The case of inelastic binary collisions is finally considered, obtaining different reduced formulas for different modelizations of the collision event. In the rest of this work we use the simplest of them, the one for the instantaneous collision with a fixed restitution coefficient and no rotational degrees of freedom, that is Eq. (2.41).