A. Thomas McLellan is the founder and chairman of the board of directors at the Treatment Research Institute, a not-for-profit research and development institute in Philadelphia, Pennsylvania. McLellan was the principal developer of the Addiction Severity Index (ASI) and the Treatment Services Review (TSR). He has served as an adviser to many government and nonprofit scientific organizations, including the Office of National Drug Control Policy, the National Practice Laboratory of the American Psychiatric Association, the Swiss National Science Foundation, the World Health Organization, the Greek government, and Public Health England. McLellan served as the deputy director of the Office of National Drug Control Policy under the Obama administration.