Abstract
We study the semi-classical limit of the dynamics of electrons in a crystal under the influence of an external electric field. Two snlall parameters E and h are introduced. They are respectively related to the scaled lattice length and the scaled Planck constant. When ε/h and h go to zero, we prove that in the limit process the dynamics of electrons is described by a Vlasov equation. When h is fixed and ε tends to zero, the dynan~ics is governed by an effective Scluljdinger equation. In both cases, the mass of electrons is replaced by a matrix, the so-called effective mass.