Wikis have become enormously attractive to Internet users because they are open-access web pages or networks of web pages that can be modified by any interested editors, making them perpetual works-in-progress that evolve and change at the behest of their contributors. Wikipedia, the limitless fountain of collected, and sometimes inaccurate, information, is now the largest encyclopedia in the world. Its popularity suggests not only that digital literacy demands participation and collaboration, but also that our current reading practices and information gathering are highly encyclopedic and reconstructive. This essay argues that wikis have origins in literate practices that predate the age of print, specifically in medieval encyclopedia writing. From Isidore of Seville to Honorius Augustodunensis to William Caxton, understanding the world meant creating and recreating its image, Imago mundi, in a language that would be accessible to more and more readers. Since students are using and misusing Wikipedia with increasing regularity as a foundation for learning in all of the disciplines, it is important for educators to understand the history of this collaborative, yet contested, literate behavior.