Overview of social software; what are wikis (an introduction to wikis and an overview of how it is quick and easy for users to edit them); common wiki features (web access, ease of editing, ease of adding links, real time update, collective authoring, versioning, recent changes, alerts, search and navigation, simple permissions structure); wiki philosophy; wikis as social information spaces that combine technology, space, information resource, philosophy and group or community; a brief history of wikis; wiki limitations, criticisms and controversies; wiki vocabulary, including definition of wiki and other terms; a survey of how wikis are used. Featured uses of wikis are: reference works; websites for communities of practice, hobbyists and other interest groups; technical documentation and standards; directories and lists; resource sharing; news reports; conferences and events; collaboration on projects and in committees; surveys; other shared authoring projects; website development; personal wikis. The chapter touches on how wikis differ from other social software including blogs, email and groupware, and soft security and community in wikis.