This thesis focusses on the self-reported reasons for cannabis use among people with psychosis, what is wanted from a psychological intervention, including the treatment preferences within this group. Paper 1 provides a systematic review of the self-report literature on reasons for cannabis use among people experiencing psychosis. Fourteen studies were identified that satisfied inclusion criteria for the review and discussed using a narrative synthesis. The most commonly reported reasons for cannabis use were organised under three themes: to escape from or cope with negative affect, to enhance positive affect, and social reasons. The strengths, limitations and effectiveness of the literature as a whole are considered throughout the review and recommendations for future research are made. Theoretical and clinical implications are also discussed.Paper 2 used both quantitative and qualitative research methods to investigate the treatment preferences of cannabis users with psychosis. Thematic analysis of the data revealed two themes: Motivation to change behaviour, with subthemes Motivation to change cannabis use and Motivation to engage with services; and The ideal approach to treatment, with subthemes Preferred qualities of support and Preferred treatment outcomes. Mixed views and experiences were described. Preferences for treatment included: for readiness to change to be considered, to be involved in treatment decisions (regarding type, delivery and goals of treatment), and development of practical skills through psychoeducation and physical health interventions. Clinical implications and recommendations for future research are discussed.Paper 3 is a critical reflection of the submitted papers and research process as a whole. The strengths and limitations of the presented research, methodological considerations and implications for clinical practice and theory are discussed and directions for future research are highlighted. In summary, this is the first study to explore treatment preferences in cannabis users with psychosis, providing detailed exploration of why people with psychosis use cannabis and what they would like from a psychological intervention.