Wikis are commonly perceived as online collections of content. From Wikipedia, the largest reference work ever created, to smaller topical wikis about entertainment, home repair, travel, news, and countless other themes, wikis have been used to make an extensive array of information easily available online. The purpose of Wikipedia—to make high-quality encyclopedic content freely available to everyone in the world in all languages—is an ambitious educational goal, but making content widely available is only one of the ways by which wikis have proved to be a radical force in both in-school and out-of-school learning. This entry further defines wikis and discusses their development and their connection to learning.