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Hepatitis C virus (HCV) is a bloodborne virus that infects the liver, causing significant morbidity and mortality. It has been declared a global health threat due to its impact on the lives of 130-150 million people who are chronically infected. For most of the Western world, HCV is the primary cause of end-stage liver disease, liver cancer, liver transplant, and liverrelated death. A rise in morbidity and mortality from hepatitis C is expected to peak in the next decade as complications have a latency of 20-30 years from disease onset, and the first peak of the hepatitis epidemic occurred in the 1990s among the baby boomer age group. Adherence and completion of treatment for patients with hepatitis C remains a challenge. While biomedical (medical and physiological) host determinants explain approximately 50% of the variance in treatment outcomes, the key psychosocial (psychological and social) factors have not yet been unequivocally established. With interferon-based treatments, uptake, adherence, and treatment completion have been suboptimal. Even in the new era of improved interferon-free treatment regimens, psychosocial factors could remain a significant barrier to adequate medication adherence, particularly in difficult to treat populations, such as those with substance abuse problems and unstable housing. To improve patient outcomes, the aim of this thesis was to identify the key psychosocial factors that influence patient adherence and completion through the development and testing of an assessment tool. The purpose of the assessment tool would be to determine patient readiness for treatment and identify those patients who required psychosocial intervention before treatment commencement, to improve adherence and likelihood of completion. To develop the assessment tool, a mixed method research approach was structured in three stages. In stage one, a systematic review was performed to clarify current knowledge of the 10 psychosocial factors that influence treatment outcomes for patients with hepatitis C. Stage two was a qualitative study to gain further insight into the barriers and facilitators of hepatitis C treatment adherence and completion. This involved in-depth interviews with patients with hepatitis C and the doctors and nurses who treat them. These studies informed the design of a psychosocial assessment tool, comprising standardised instruments measuring nine psychosocial factors. In stage three, this tool was tested in a longitudinal pilot study, to determine its ability to predict treatment adherence and completion and identify key areas for intervention. This project has resulted in four published papers and one submitted manuscript, which provide novel insights into the psychological and social challenges faced by patients with hepatitis C, the mechanisms they use to cope with a stigmatised disease and difficult treatment regimen, and the specific barriers and facilitators they experience on their treatment journey. Based on our findings, I have proposed a model linking key factors that influence treatment outcomes. Finally, this thesis provides insights into fruitful areas for future research into treatment adherence among patients with hepatitis C, with potential broader implications for patients with other chronic diseases