In late May and early June 1991, a NATO Advanced Study Institute was held at a hotel in the hilltop village of San Martino al Cimino a few kilometers from the city of Viterbo in the Lazio region of Italy. The title of the course was the same as this volume and brought together specialists working at all phases of the life span (embryology, infancy, childhood, middle life and senescence) in both animals and humans to exchange ideas, facts and theories in the search for common principles. Such principles could prove important for understanding developmental changes in the central nervous system and visual behavior within the context of a continuum of life-span processes rather than viewing them as events or mechanisms that occur only in a certain period. For example, changes that are associated with "aging" were considered as extensions or continuations of processes that began at an earlier stage of the life span, rather than being seen as processes that only began late in life.