This ebook focuses on the co-occurrence of mental and substance use disorders among young people (under 25 years). Half of all lifetime mental disorders develop prior to the age of 14 years, and three-quarters by age 24, making them the leading cause of disability in young people worldwide. Young people are also at high risk of experiencing comorbidity across disorders. The articles in this issue provide an important contribution to our understanding of the relationship between mental and substance use disorders among young people. In particular, these papers provide an examination of emotional and other psychosocial risk factors associated with risky or harmful alcohol consumption and cannabis use, and particular at risk groups. Their findings highlight the need for universal and targeted prevention efforts to halt the progression of harmful use and the development of primary or secondary disorders, and the need for age appropriate psychosocial interventions.