In this thesis, I explore the impact and effectiveness of clinical supervision provided by external clinical supervisors, on workers from a variety of practice backgrounds in a residential rehabilitation centre, in the AOD sector. This thesis uses the framework of social work supervision, as defined by Kadushin (1985; 2002), with the administrative, educative and supportive elements. The research focuses closely on the latter two elements. For this purpose, I employ qualitative research methods, via a triangulation of methods, being guided by Participatory Action Research (PAR) and then conducting semi-structured interviews and focus groups as well as acting as a participant observer, to gather the data. The data was analysed using grounded theory. This research was based upon a clinical supervision project that was fully funded and provided free clinical supervision by external supervisors, providing both individual and group supervision, for a period of 10 months. There were a total of 16 respondents with varied roles and training backgrounds as well as an additional six supervisors, interviewed throughout different stages of the project. The various roles undertaken by me throughout the research process provided essential viewpoints on supervision, as well as the place of boundaries and need for support. The power of such a large scale intervention is discussed and ultimately highlights and identifies the particular benefits of supervision in this research arena. This thesis places clinical supervision in the context of workforce development in the Alcohol and Other Drug (AOD) sector. Thus, while this research elucidates a number of benefits and the factors involved with this experience, the clear separation of other modalities such as training, mentoring and Critical Incident debriefing are seen as integral additional avenues of support and professional development. This research concludes also with what the difficulties and hindrances were for people to continue with regular ongoing clinical supervision, and warrants the argument for interminable supervision in this setting. This research points to an overall paucity of literature on efficacy studies, especially in the AOD context in Australia. This research significantly adds to this dearth and examines the factors unique to the AOD sector in Australia, as well as what factors make for effective supervision. The particular impact of external supervisors and group supervision are explored, which underwrites the forwarded concept of a customised supervision for this setting. By making explicit in this research what the efficacy is on those new to supervision, it provides greater clarity for future studies. A number of recommendations are proposed as result of this research. New definitions of the supportive function of supervision and of clinical supervision are forwarded, as is a new look at the evolving history of social work supervision. This thesis highlights the impact of external supervisors and the unique contribution they offer.