The Teen Addiction Severity Index-Two (T-ASI-2) was developed as an extension of the T-ASI to assess the severity of substance abuse and related problems among adolescents (N = 371) 12-19 years of age. The T-ASI-2 consists of 18 domains that assess current use of alcohol, tobacco, marijuana, and other drugs, as well as mental health service utilization, treatment satisfaction, school difficulties, social functioning with family members and peers, substance use by family members and peers, depression, anxiety, attention deficit, hyperactivity, defiant and risky behaviors, and readiness for change. Results show that all domains have adequate to excellent internal consistency (0.54 to 0.88, median 0.80). New domains assessing psychological factors strongly correlated with gold standard assessments in the respective areas. The T-ASI-2 was designed to be a user friendly, cost-effective, viable assessment of substance use behavior and related factors. (Contains 3 tables.)