Wikis are often considered to be the core platform of peer production. This chapter delineates their most central design principles, history and affordances. Wikis are described as content management systems that allow for flexible collaboration without a defined content owner or leader. Users can modify the content and structure of documents directly in their web browser. Edits are usually archived and open to revision. This chapter pays particular attention to the most successful wiki-based system, the non-profit, online encyclopedia Wikipedia. As the chapter explains, Wikipedia actually contains a broad range of more or less individual wiki projects and has inspired a plethora of other endeavors, both open source and proprietary. The particular peer production model employed in Wikipedia is elucidated, and in the course also complicated. The chapter concludes by highlighting a number of tensions emerging from a wiki-based peer production model, between amateurs and experts, human editors and bots, lay knowledge and academic knowledge and the shaping of trust through external actors.