The Addiction Severity Index (ASI) is a multi-dimensional interview used to measure the substance use, health, and social problems of those with alcohol and other drug problems, both at admission to treatment and subsequently at follow-up contacts. This article first discusses the conceptual and practical importance of the ASI's multi-dimensional approach to measuring addiction severity, as illustrated by two case presentations. The second section of the paper reviews how this measurement approach has led to some important findings regarding the prediction and measurement of addiction treatment effectiveness. The third section describes the historical and practical considerations that have changed the instrument over time, details the problems with the instrument, and describes our efforts to correct those problems with the ASI-6. Finally, some recent ASI data collected from over 8,400 patients admitted to a nationally representative sample of U.S. addiction treatment programs are presented.