Adolescence is a period of transition, marked by physical, psychological, and cognitive changes underpin by biological factors. Today's generation of young people--the largest in history--is approaching adulthood in a world vastly different from previous generations; AIDS, globalisation, urbanisation, electronic communication, migration, and economic challenges have radically transformed the landscape. Transition to productive and healthy adults is further shaped by societal context, including gender and socialisation process. With the evidence that young people are not as healthy as they seem, addressing the health and development issues of young people, more than ever before, need concerted and holistic approach. Such approach must take the entire lifecycle of the young person as well as the social environment into context. This is particularly critical in developing countries, where three major factors converge--comparatively higher proportion of young people in the population, disproportionately high burden of youth-related health problems, and greater resources challenge.