The DENS collects extensive background and recent status data on patients' drug, alcohol, psychiatric, medical, employment, legal, and family problems as they enter a sample of treatment programs throughout the country. The DENS recognized the need for descriptive information on important structural, organizational, and service delivery aspects of the programs in which those patients were treated. Toward this end, the authors present their efforts thus far in characterizing and monitoring "service delivery units" or "programs" that are sampled in the DENS system. Specifically, this paper describes the development of the Addiction Treatment Inventory (ATI), a standardized measurement instrument to characterize these service delivery units and their services. The ATI focuses on organizational structure, patient profile, service profile, staffing mix, and financing. Although the ATI does collect substantial information on treatment programs, the authors acknowledge that it has limitations. Most importantly, it is limited by reliance on the knowledge, availability, and resourcefulness of the staff member completing it. It is at best descriptive, and it is not possible to infer indications about the quality or appropriateness of staffing patterns or service delivery patterns from the ATI. 4 tables, 1 figure, and 15 references