Background. Mindfulness-based Cognitive Therapy (MBCT) is a relapse-prevention treatment for people in remission from major depression (rMDD). The neuropsychological mechanisms of MBCT are largely unknown. The key theoretical mechanisms of MBCT include self-compassion and rumination, with other mindfulness-based studies suggesting mechanisms across domains of attention, emotional processing, and cognitive flexibility. The aims of this thesis were to investigate the neuropsychological and neuroimaging mechanisms of MBCT in rMDD and to investigate relationships with relapse six months beyond MBCT. Paper One. 40 MBCT, 33 Treatment As Usual (TAU), and 42 healthy volunteers (HVs) took part. Experimental tasks for self-compassion and rumination were completed at baseline and post-session, alongside self-report questionnaires. Results showed increases for self-compassion following MBCT, with non-specific reductions for rumination. Paper Two. 40 MBCT, 33 TAU, and 42 HV participants took part. Tasks of attention, emotional processing, and cognitive flexibility were completed at baseline and post-session. Results showed increases in positive emotions post-MBCT, with non-specific changes for face emotion recognition and affective attentional bias tasks both post-MBCT and TAU. There were no changes over time for sustained attention or cognitive flexibility. Paper Three. 16 rMDD participants completed baseline and post-MBCT fMRI tasks of sustained attention and self-blame. Results showed reduced activation over time in the bilateral dACC in self-blame contrasts but no activation changes for sustained attention. Paper Four. 69 rMDD participants (38 MBCT & 31 TAU) completed task-based and self-report assessments up to six months follow-up. Non-relapsing MBCT participants showed increased self-compassion post-session which was maintained in follow-up. MBCT non-relapsing and TAU participants showed reduced rumination post-session and in follow-up. Conclusions. MBCT encourages a move towards more self-compassionate, positive processing but does not alter more automatic, bottom-up levels of processing. MBCT initiates a process for change beyond the course, particularly for self-compassion. Findings have theoretical and clinical implications and extend our understanding of the mechanisms of MBCT in rMDD participants.