If we consider the male, female and homosexual as the first, second and third sex, the fourth sex might be that of adolescents. But adolescence is above all a sexually undefined state. Teenagers are not little boys or little girls, and they are not yet men or women. They are part of a parallel, fluid universe in a state of becoming. They belong decisively to the present, but in symbolic terms they are the seeds of the future. Adolescence is also a state of mind, an existential condition with a powerful impact on lifestyles and trends. Teenagers are omnivorous, tireless consumers, careless but at times attentive, easily influenced but autonomous. Off balance between the present and the future, they seem overexcited and at the same time strangely passive. They often give form to their world and their culture in an aggressive manner, but at the same time they are forced to come to grips with the labels, judgments and formulae of adults. Their behaviour patterns are constantly monitored because they represent a decisive segment of the strategy of consumption. Fashion pays particularly close attention to the teenage universe both as a source of inspiration and as a crucial consumer segment, while contemporary art probes, exploits and analyses the myth of the eternal adolescent. This book is constructed with images of fashion, art, music, cinema and the news, accompanied by an anthology of writings that mixes poetry, literature, journalism and research, highlighting the increasing importance of the teenage tribe in a society such as ours, where the inadequacy of sexual and demographic identities is reflected in a violent, contradictory way in the social changes in progress.