Legislators, journalists and concerned citizens in general, when consider- ing what to do about the plague of heroin addiction in large cities, ask an obvious question: "Is methadone treatment effective?" This question is a critical one since maintenance with methadone is at present the only prac- tical alternative to leaving tens of thousands (in New York City, hundreds of thousands) of untreated addicts on the streets. Other treatments, although effective for limited groups, could not conceivably be expanded to stop heroin use in as much as 10% of the addicted population. The present study, sponsored by the National Institute on Drug Abuse, was undertaken to provide an authoritative answer to this question. Under the direction of a distinguished expert, the evaluation team made an inten- sive examination of techniques and outcomes in six different methadone programs located in New York, Philadelphia, and Baltimore, and followed this by two yhears of data analysis and literature review. The present re- port is the product of this work. The primary conclusion-namely that methadone treatment is substan- tially effective in reducing heroin use and associated criminal behavior-is consistent with the findings of several previous independent evaluations.