This study involved the evaluation of the Alcohol and Drug Advice Centre in Swansea, examining not only its general effectiveness but also the theory underlying its practice. The fieldwork took place over a period of two years and six months, 1986 to 1988. The aim was to do a small scale intensive study, which would give insight into the process of treatment, change and maintenance of change, and provide an understanding of individual and interpersonal processes taking place at the Centre. An ethnographic study of forty clients as they passed through the treatment process facilitated the examination of possible variables influencing different outcomes, giving an indication of why particular individuals 'succeeded' in treatment while others 'failed'. The qualitative data collected give an indication of the degree of compatibility between views of clients and the beliefs of staff together with the Alcoholics Anonymous philosophy of the Centre. These data were supplemented by a statistical analysis of client characteristics and attendance. The theory and practice of the Centre is placed in the context of both historical and contemporary theories of addiction. Theoretical controversies and changes in policy and practice are reviewed. The work in this study has demonstrated that there is some correlation between the beliefs and theories of client and therapist on the one hand and successful outcome on the other. The implications of these conclusions for treatment are discussed, focusing on the possibility of pre-treatment 'matching' of clients to particular approaches. It is suggested that different theoretical components that both inform and partly constitute experiences of treatment should be evaluated, not only in terms of internal consistency and external validity but also in terms of usefulness and therapeutic tools or ideologies. The possibility of combining conflicting treatment theories within the provision of an effective service, by a separation of allocation and treatment functions, is considered.