Substance use among youth presents an urgent public health problem, associated with the leading causes of morbidity and mortality. National data show that: 28.2% of high school seniors report past-month binge drinking (5 or more drinks); 20.8% of young adults and 6.5% of adolescents report past-month cannabis use; and nearly one in five youth report non-medical use of prescription opioids. Mobile health (mHealth) allows for more accessible and cost-effective substance use prevention and treatment; yet, few mHealth approaches exist which target substance use among adolescents/emerging adults. This project will develop and test a mobile phone app to deliver just-in-time adaptive interventions targeting youth substance use.