The tondo, one of the most popular formats for devotional painting and sculpture in Quattrocento Florence, reflected specific religious and cultural ideas, in particular humanism and its veneration of the circle. The documentary material presented here contains clues not only as to the origins of the tondo but also regarding the meanings it held in the Renaissance. This synthetic study opens with an examination of the philological background of the word "tondo," and goes on to discuss various inventories in detail, including those of the Medici family and the Magistralo dei Pupilli, as well as commission and payment documents, and artists' inventories. In the final analysis, an astonishing correlation exists between the documents and the physical evidence. The author also assesses both written records and works of art to determine where tondi would have been placed.