What is the story behind the sea monsters seen on so many early European maps? Their first appearance can be traced back to 10th century mappaemundi and continue through the end of the 1500s. They are depicted in various forms – swimming vigorously, gamboling amid the waves, or attacking ships – and are one of the most visually engaging elements on these maps. These sea monsters are important not only in the history of cartography, art, and zoological illustration, but also in the history of the geography and of western conceptions of the ocean. They can also supply important information about the sources, influences, and methods of the cartographers who drew or painted them.