The first chapter consists of a systematic review of the literature concerning the construct of Experiential Avoidance in relation to Substance Use Disorder. A comprehensive database search was conducted which resulted in 16 studies then included in the review. Positive findings supported the role of Experiential Avoidance in various aspects of Substance Use Disorder including risk factors, comorbidity with other mental health problems such as post-traumatic stress disorder, success in treatment and abstinence. However, there were also many inconsistent and contradictory findings. Several limitations with the literature were found including an over-reliance on cross-sectional designs and self-report measures. Issues surrounding the measurement of Experiential Avoidance are also outlined. Due to these inconsistencies and issues with the research, it was not possible to draw firm conclusions. Implications for future research are discussed.

The second chapter is a qualitative exploration of Moving On In My Recovery: a new, Acceptance and Commitment Therapy (ACT) based group intervention for Substance Use Disorder. Grounded theory was used to build a model of the process of change towards recovery in the group, grounded in the participants’ experiences. Ten participants were interviewed, who were abstinent from substances following engagement in the group. The model that emerged depicted a chronological series of processes centring around the core category of reinforcement from engaging with the group and recovery-consistent behaviours. Other processes reflected group-based factors which contributed to sense of safety, and individual factors such as investment in recovery, which supported engagement with core ACT therapeutic processes. Clinical implications and suggestions for future research are discussed.

The final paper provides discussion of the implications for theory development, future research and clinical practice arising from both the literature review and research papers. The thesis ends with a reflective commentary on the research process.