Martin Waldseemüller created two large world maps in 1507 and 1516, just nine years apart, but the two maps could not be more different: while the 1507 map is based on Ptolemy’s Geography, in his 1516 Carta marina he sets aside the Ptolemaic model and adapts that of nautical charts instead. Moreover, the cartographer abandons almost all of the sources he used in creating his 1507 map, and undertook detailed research in contemporary geographical texts and maps for the details he wanted for his new map. We have very little information about the workshop practices of early sixteenth-century cartographers—about how they created their maps. Waldseemüller’s Carta marina of 1516 offers a rare opportunity to obtain just this sort of information. By examining the cartographer’s sources, both cartographic and textual, as well as how he used those sources, we can reconstruct how he went about creating this magnificent map, and gain a unique and unprecedented view of an early modern cartographer at work. The talk also includes evidence regarding the diffusion of the Carta marina.