Although photovoltaic industry rapidly grows, cost reduction is the most important issue for spread of photovoltaics. Various technical issues for crystalline or thin-film Si solar cells and modules will be discussed in detail. Those for compound thin-film such as Cu-In-Ga-Se (CIGS), dye-sensitized and organic thin-film solar cells will be also introduced. Reduction for Si use is key technology for cost reduction of crystalline Si solar cells. Various methods are developed for improved properties of thinner wafer crystalline Si solar cells. Large-area and high rate deposition of microcrystalline Si films are key technology for multi-junction thin-film Si solar cells. Various polymer materials are used in module-formation processes and those polymer materials determine the reliability of modules. New type solar cells such as CIGS, dye-sensitized and organic thin-film ones are of course necessary for dramatic cost reduction of photovoltaics.