Theophilus Presbyter (flourished c. 1070–1125) is the pseudonymous author or compiler of a Latin text containing detailed descriptions of various medieval arts, a text commonly known as the Schedula diversarum artium ("List of various arts") or De diversis artibus ("On various arts"), probably first compiled between 1100 and 1120. The oldest manuscript copies of the work are found in Vienna (Austrian National Library, Codex 2527) and in Wolfenbüttel (Herzog-August-Bibliothek, Cod. Guelf. Gud. Lat. 69 2°). Gotthold Ephraim Lessing rediscovered the document when he worked as librarian in Wolfenbüttel, and published excerpts in 1774. These aroused great interest as they disproved Vasari's myth of Jan van Eyck developing the technique of oil painting in the early 15th century, about which antiquaries had already become suspicious.
Theophilus' Schedula allows detailed insights into the techniques used in the applied arts in the high Middle Ages. The work is divided into three books. The first covers the production and use of painting and drawing materials (painting techniques, paints, and ink), especially for illumination of texts and painting of walls. The second deals with the production of stained glass and techniques of glass painting, while the last deals with various techniques of goldsmithing and other metalwork. It also includes an introduction into the building of organs. Theophilus contains perhaps the earliest reference to oil paint. The work has been translated into English, French, Polish, Portuguese, Spanish, Hungarian, German, Italian, Japanese, Bulgarian, and Russian, mostly in the 19th and 20th centuries.
'Theophilus' was quite possibly a Benedictine monk. It has been suggested (particularly by Eckhard Freise) that Theophilus is the same person as the artisan monk Roger of Helmarshausen. Roger appears to have come from Stavelot Abbey in the Meuse River region, was active as an artist and author between 1100 and 1107 in St. Pantaleon's church in Cologne, and moved to Helmarshausen Abbey in 1107. The identity of the two men has been argued among researchers for some time, but Freise's conclusions have not yet been accepted by all researchers. Other suggestions have also been made, and at present there can not be said to be a consensus. 
Theophilus, as the author of a "handbook", has been described as a mere theorist, but this view is currently in the minority. The First Book, on painting, is not particularly well-informed, but adequately reliable, the Second, on glass, is better, while most of the Third Book is clearly the work of a practising metalworker. It has recently been suggested that the apparent contradictory evidence as to dating, practical experience, and location of 'Theophilus' is best explained if the Schedula is understood to be a compilation.
- De diversis artibus or Schedula diversarum artium (in three books, date controversial but ca. 1125)
Two editions of Theophilus's work with English translations:
- Dodwell, C.R. The Various Arts. De Diversis Artibus. Oxford: Clarendon Press, 1961, reprinted 1986
- Hendrie R. An Essay upon Various Arts in three Books by Theophilus called also Rugerus. London, 1847, 1961
A good English translation:
- Hawthorne, J.G. and C.S. Smith Theophilus: On Divers Arts. University of Chicago Press, 1963; reprinted New York: Dover Publications 1979; ISBN 0-486-23784-2
Reference in English:
- White, Lynn Theophilus redivivus. Technology and Culture 5, 1964, 224-233; reprinted in Medieval Religion and Technology, University of California Press, 1978.
- Thompson, Daniel V., 'Theophilus Presbyter, words and meanings in technical translation'. In Speculum 42/2 1967, 313-339.
The following references are in German:
- Brepohl, E. Theophilus Presbyter und das mittelalterliche Kunsthandwerk. Cologne, 1999 (2 volumes).
- Freise, E. "Roger von Helmarshausen in seiner monastischen Umwelt". In Frühmittelalterliche Studien, 15, 1981.
- Ilg, A. (Editor). "Theophilus Presbyter. Schedula diversarum artium". In Quellenschriften für Kunstgeschichte, volume 7. Vienna, 1874.
- Lessing, Gotthold Ephraim. Vom Alter der Ölmalerey aus dem Theophilus Presbyter. Berlin, 1774.
- Theobald, W. Technik des Kunsthandwerks im 10. Jh. Des T. Schedula Diversarum Artium, Berlin, 1933, 1953 and 1983 (includes translations and explanations of sections of the work).
- Incomplete Latin e-text.
- Hendrie's edition and English translation of De diversis artibus, at Google Books
- English and French medieval stained glass in the collection of the Metropolitan Museum of Art, an exhibition catalog from The Metropolitan Museum of Art (fully available online as PDF), which contains material on Theophilus Presbyter
- Gotlieb, Marc, "The Painter's Secret: Invention and Rivalry from Vasari to Balzac", p. 473, The Art Bulletin , Vol. 84, No. 3 (Sep., 2002), pp. 469-490, JSTOR
- , "Around Theophilus": expert meeting, Wolfenbüttel 2010.
- Clarke, M. (2011) Mediaeval Painters’ Materials and Techniques: The Montpellier ‘Liber diversarum arcium’. London: Archetype Publications: 56-7