Substance use in young people (aged 10–24 years) is a significant health and social problem throughout the world. The 2013 Global Burden of Disease estimated that substance use among young men was responsible for 14% of total health burden. Recent findings have underlined the neurobiological vulnerability of young adults and the impact of co-occurring substance use disorders and mental health problems that emerge in adolescence. Adolescence is the peak period for initiation of substance use. A consistent finding from the international literature is that levels and frequency of use of drugs, alcohol, and tobacco begin to increase in mid-adolescence and peak in very early adulthood. These are key periods of change, cognitive and emotional development, and transitioning into employment, stable relationships, and parenthood.