This volume includes sixty-two papers presented in the MRS Symposium A, "Amorphous and Polycrystalline Thin-Film Silicon Science and Technology-2012," which took place on April 9-13, 2012, in San Francisco, California. The symposium covers the science and technology of thin-film silicon based materials and devices. Among various applications, solar cells for photovoltaic energy conversion and thin-film transistors for flat-panel displays have been the two major driving forces for research and development of thin-film silicon materials and devices. In the past few years, the thin-film silicon community has mainly focused on thin-film silicon solar cells to address the issues of efficiency, manufacturing capability and manufacturing cost. This year Symposium A held a special session focusing on industry.