From the ages of ten to nineteen, adolescence is the transitional period between childhood and maturity. It is a unique stage of human development and a critical period for setting the groundwork for optimal health. Adolescents grow rapidly in terms of their physical, cognitive, and psychological development. This has an impact on how individuals feel, think, make decisions, and interact with others. Despite its reputation as a healthy period of life, adolescence is marked by major death, disease, and damage. Much of this may be avoided or treated. Adolescents form patterns of behaviour – such as those linked to food, physical exercise, substance use, and sexual activity – that can either safeguard their health and the health of those around them, or put their health and the health of others at danger now and in the future.