This book is primarily concerned with the youthful use and misuse of alcohol, tobacco and illicit drugs and with the sexual behaviour of young people. As a preliminary to the presentation of a selective review of some of the evidence related to these topics it is considered important to provide readers with a brief introduction to the many possible reasons why people sometimes use and misuse psychoactive (mind-altering) drugs. Such drugs, it should be noted, include alcohol, tobacco and prescribed as well as illicit drugs.