Cosensitization, a technique involving sensitization of a metal-oxide semiconductor electrode with two or more different dyes, is a promising strategy to enhance light-harvesting capabilities of dye-sensitized solar cells through expansion of the light absorption range. In this review, we introduce recent developments in highly efficient solar cells cosensitized by ruthenium polypyridyl and organic dyes. Design strategies for cosensitizer molecules, development of UV and NIR cosensitizers, and performances of reported cosensitized solar cells are reviewed. Cosensitization is an effective method to improve the conversion efficiencies, as not only photocurrent but also photovoltage can be increased by employing an appropriate cosensitizer.