The Drug Treatment Outcomes Research Study (DTORS) is the more recent of two such major English studies. Over the past decade, a substantial policy shift has emphasised the role of treatment in diverting drug misusing offenders from the criminal justice system. At the same time, changes in the drug-using population, particularly increasing levels of crack cocaine use, have led to an acute need to evaluate the extent to which drug treatment works. DTORS consists of three components: the main, quantitative element tracking the treatment progress of N=1,796 drug treatment seekers; qualitative work using data from a sample of treatment seekers and treatment providers; and a cost effectiveness analysis. This paper provides an overview of the methods used for the main quantitative element; a longitudinal, nationwide, multisite, observational, cohort study using survey interviews to assess the impact of treatment on drug use, offending, and health. An overview of the DTORS baseline sample is provided and compared with the target population of drug treatment seekers in England. Participants were drug users seeking treatment from community based or residential treatment services, assessed at baseline and followed up over a 12-month period. The baseline DTORS sample was predominantly male (73%) and aged 25 to 34 (47%). Primary problem drugs included heroin (57%) and crack (12%) with drugs used in the month prior to baseline having a mean value of £1,213. 72% committed offences in the 12 months before recruitment, most commonly shoplifting or buying and selling stolen goods.