This article discusses the implementation of the Drug Evaluation Network System (DENS) version of the Addiction Severity Index (ASI) among substance abuse treatment providers in Los Angeles County. Thirty-two trained and certified treatment professionals from 14 programs participated in focus group interviews about their use of and experiences with the ASI. All 14 programs were currently administering the ASI at the time of the study. Qualitative interviews revealed variation among the programs in terms of when staff administered the ASI and how they used it in clinical care. Although the ASI creates a comprehensive picture of substance-abusing patients and their functioning, the timing of its administration seems to significantly affect the usefulness of the information. The findings illustrate the importance of practicality, flexibility, and ongoing training for the successful implementation of evidence-based practices.