The sea monsters on medieval and Renaissance maps, whether swimming vigorously, gambolling amid the waves, attacking ships, or simply displaying themselves for our appreciation, are one of the most visually engaging elements on these maps, and yet they have never been carefully studied. The subject is important not only in the history of cartography, art, and zoological illustration, but also in the history of the geography of the `marvellous` and of western conceptions of the ocean. Moreover, the sea monsters depicted on maps can supply important insights into the sources, influences, and methods of the cartographers who drew or painted them. In this highly-illustrated book the author analyzes the most important examples of sea monsters on medieval and Renaissance maps produced in Europe, beginning with the earliest mappaemundi on which they appear in the tenth century and continuing to the end of the sixteenth century.