Among various applications, solar cells for photovoltaic solar energy and thin-film transistors for flat-panel display have been the two major driving forces for research and development of thin-film silicon materials and devices. In the last 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 volume includes sixty-eight papers presented at Symposium A, “Amorphous and Polycrystalline Thin-Film Silicon Science and Technology—2011,” which took place April 25-29, during the 2011 MRS Spring Meeting in San Francisco, California. The symposium covered the science and technology of thin-film silicon based materials, reviewed recent progress, addressed the scientific and technical issues in the field, and this year, focused sessions were held on the topic of solar cell efficiency.