The low material cost and proven manufacturability of amorphous silicon (a-Si) alloy solar panels make them ideally suited for low-cost terrestrial application, A major challenge for the researchers has been how to improve the light-to-electricity conversion efficiency. Extensive R & D efforts have resulted in a significant improvement in stable cell and module efficiencies with the achievement of 12.8% active-area cell efficiency and 10.4% module efficiency using a spectral-splitting triple-band gap, triple-cell approach. Further gains in efficiency are expected through an improved understanding of plasma chemistry and growth kinetics. In this paper, we shall discuss the progress in science and technology of a-Si alloy photovoltaics with special emphasis on the opportunities and the challenges that exist.