This volume had its origin in lectures and seminars presented in Rome and Ann Arbor in 1978. Since that time a number of books and articles have been written on the subject, and I have tried to take the most important ones into account here. A reference list follows each chapter. I would like to thank the Thomas Spencer Jerome Committee for the precious help granted in Rome and in Ann Arbor, and numerous friends in the Department of Classics at the University of Michigan and of the Kelsey Museum who have read and corrected my English text. I am particularly grateful to various colleagues mentioned in the notes for suggestions and advice.