There are 1.2 billion adolescents in the world, more than double the number in 1950, and the largest number the world has ever known. Almost 90% of adolescents live in low- and middle-income countries. 16 million girls aged 15-19 years become pregnant every year. Most young people are healthy. However, globally more that 2.6 million young people aged 10 to 24 die each year. A much greater number of young people suffer from conditions that hinder their ability to grow and develop to their full potential. A greater number still engage in behaviour that jeopardizes not only their current state of health, but often their health for years to come. Nearly two-thirds of premature deaths and one-third of the total disease burden in adults are associated with conditions or behaviour that began in youth, including: tobacco use, alcohol and drug abuse, a lack of physical activity, unprotected sex or exposure to violence and injury.