Ever since humankind first ventured out onto the oceans, sailors came back with stories of sea monsters. For two hundred years, scientists have been attempting to classify these 'creatures' within an acceptable zoological frame of reference. The most important of these was produced by Professor Bernard Heuvelmans half a century ago. Michael Woodley, takes a look at Heuvelmans' classification model, re-examines it in the light of new discoveries in palaeontology and ichthyology over the past fifty years, and reaches some astounding conclusions.