A "wiki" is a class of Web site that lets authorized users create and modify pages (sometimes called articles) as well as search for information. The term also sometimes describes the software that makes the site possible. The most famous example of a wiki is Wikipedia, an online encyclopedia with articles that are written and edited by site visitors. This four-year-old experiment in creating an open-source reference "book" houses more than 1.5 million articles (about 700,000 in English) and is growing daily. Educators at all levels are finding ways to incorporate wikis into their teaching. Wikis provide many exciting classroom applications. Most of all, they allow educators and students to collaborate with each other and the world in ways they never could before. This article describes three options for creating wikis.