The prevalence of substance use and substance use disorders (SUDs) and the co-occurrence of SUDs with other mental health disorders peaks in emerging adulthood. This review examines prevalence as a function of gender, race/ethnicity, historical trends, and geographic regions across both the US and Western world. Prospective predictors reviewed include the effects of early life stress, parental factors (including parental use, support, and parenting skills), peer affiliations, internalizing and externalizing behaviors, educational attainment, personality, and timing of pubertal development. Concurrent predictors include assumption of adult roles and college attendance, stress associated with life events, changes in personality, and laws and taxation. Also reviewed are consequences of use, including neurological changes. The peak in prevalence across emerging adulthood may be due to several factors, including freedom from constraint, increased peer pressure, less than optimal decision-making skills, high disinhibition, and increased stress during this developmental period.