This paper has two interrelated themes. It looks at how the idea of ‘treatment’ has historically been substance specific – and at how and why ‘evidence’ has become important in establishing the validity of the idea of treatment. For the idea of treatment, it contrasts the long and chequered history of treatment for alcohol and drugs, beginning in the 19th century, with the relatively short and rather different history of smoking treatment. So far as evidence is concerned, it uses a number of case studies of treatment research in the post-war period to examine the issues which have made the relationship between research and treatment policy increasingly central.