The overarching aim of the thesis was to contribute to the growing field of personalised medicine for depression, which aims to optimise an individual's response to treatment on the basis of their unique characteristics and underlying mechanisms. Factors that predict response to therapy, particularly theorised processes, are crucial to the development, refinement and improvement of current psychotherapies. This thesis explored the role of cognitive and interpersonal factors and their relationship with treatment outcomes during psychological treatment for depression, with a specific focus on Cognitive Behaviour Therapy (CBT). Part II is a Systematic Review, which aimed to synthesise the existing literature on change in interpersonal functioning during CBT for depression, and its association with treatment outcomes. Part III is an Empirical Study, which explored the utility of measures of dysfunctional attitudes and interpersonal problems, as pre-treatment predictors or moderators of treatment outcomes in individuals receiving CBT or Interpersonal Therapy (IPT) for depression. Part IV integrates the findings from Parts II and III, provides reflections on the process and discusses the impact of the study and plans for dissemination.