Use of psychoactive substances usually starts in adolescence and even childhood. The earlier substance use starts, the greater the risks for more rapid progression to heavy use and substance use disorders. It is important to remember, though, that the highest proportion of adolescents typically fall into a low-use group, which at times includes experimental or low levels of alcohol or drug use. Even at a low level, however, alcohol and drug use can affect normal development, result in a range of negative health and social outcomes and can lead towards the development of substance use disorders. Higher risk of poor outcomes might be associated with use of multiple substances, with those in lower socioeconomic groups and older age groups at highest risk. Attention should be given to preventing the initiation of alcohol and drug use among children and adolescents, to helping them quit substance use and to reducing its negative consequences, while addressing the special needs of this group.