We survey the problem of finding steady solutions for the Boltzmann equation. Then we present a proof that the Boltzmann equation in a slab, with boundary conditions of diffuse reflection and mass conservation at the wall and assigned total mass (per unit area) in a slab has at least one positive solution. The physical model is provided by Maxwellian molecules with angular cutoff. The equation is rewritten after a change of variables which is shown to be invertible.