This book takes its origin from material I have been using for many years during my Greek Mythology courses for the program Paideia (a collaboration between the University of the Aegean and the Center for Hellenic Studies Paideia at the University of Rhode Island) during the past eleven years. Students have always expressed special interest in the concept of monstrosity in Greek mythology and they were always keen on details about the sources of the stories. Questions about the monsters mentioned in myths came up during classes at the University of the Aegean, in the Greek Philology classes, and in the classes about Ancient Greek Theater, at the Open University. Often people wondered about the development of a story. Which was our oldest source? For how long have people been interested in the story? How has it changed over time? I tried to answer these questions, by referring to some examples borrowed from the countless stories about monsters found in myths. I am thankful to all of these students for their enthusiastic response to these stories.