The Lorentz gas is a gas of point particles that move freely in some environement of spherical obstacles, with elastic collisions at the surface of the obstacles. In 1905, H.A. Lorentz considered this simple dynamical sytem as a model for the motion of electrons in metals,and proposed to use the methods of kinetic theory to describe it in some appropriate scaling limit. While this has been rigorously justified by Gallavotti (1972) in the case of a random distribution of obstacles, the case of a periodic distribution of obstacles remains to be understood. This talk will review the latter case.