The basic idea of a wiki is to be open to everybody. The content of a wiki should be changeable as easily as possible by everybody who is willing to participate. An open wiki could grow quickly and would benefit from the ideas and work of all users, not only from the work of the webmaster. This idea and method of work has something magic and amazing about it and could be heaven if not for… well, if there weren't some evil elements in the world.
An open website is often an invitation to destructive people, whose one and only aim is to destroy other people's work. Mostly this is not a big problem (not currently) because the count of supporters is much higher than that of those evil elements, and some security functions like backup old revisions do their part to protect the wiki content and prevent data losses.
Nevertheless, there might be good reasons to limit access to the wiki. Maybe the wiki should support a closed user group and nobody outside this group should have access, or some pages should only be writable by certain people – who knows. For those tasks Dokuwiki supports a very powerful access control also known as an ACL (Access Control List) which will be described in this chapter.