The Mediterranean portulans knew a greater success than the small number of surviving manuscripts may have led people to think. These technical aids quickly spread to milieux well beyond those interested in their daily navigational use and they became tools for the preparation of the voyage and companion volumes for travellers with different interests: royal officers preparing crusading expeditions, merchants, pilgrims. The chapter describes to treat medieval portulans in a different fashion, first by examining the different problems raised by this type of document of great importance excluding the Atlantic descriptions, and then by showing how contemporary geographers or authors of travel literature used the portulans to gain knowledge of the regions they described, especially the Greco-Byzantine world. There is only one general work on the portulans, written by the historian of geography, Konrad Kretschmer. Published almost a century ago, the work betrays its time by its desire to explain the origins of the genre.