Silicon still is the material of choice for more than 90% of PV production, although very recently the commercial production of thin film PV has accelerated dramatically, partly due to a shortage of silicon feedstock. We provide an overview of the physics of three thin-film materials, amorphous silicon, polycrystalline CdTe, and polycrystalline CuIn(Ga)Se-2. Efficiencies now challenge or exceed those of multicrystalline Si in small devices and the expectation is that production costs will soon be significantly lower than for silicon. We discuss some of the opportunities and challenges for materials scientists and engineers in these three classes of materials. (c) 2006 Elsevier B.V. All rights reserved.