This paper presents the analysis from a qualitative study of two substance misuse treatment agencies, one based on the Twelve Step AA model and one on a Cognitive Behavioural approach. Analysis of ethnographic data and 38 semi-structured interviews with staff indicated that the agencies were in practice using very similar methods. However, the use of particular therapist models gave rise to different definitions of problems, assessment and notions of success. The paper considers the implications of these findings for policy and practice. It distinguishes between the therapeutic usefulness of theory and its role in focusing assessment procedures, limiting interventions, defining success and determining the timing of treatment termination. It considers how far models might influence purchasing and service development through research and evaluation.