This thesis provides a comprehensive review of metacognitive therapy (MCT) for major depressive disorder, synthesizing evidence on the cognitive and metacognitive mechanisms underlying depression. Depression reflects not only cognitive impairments such as slowed processing and weakened executive control, but also maladaptive metacognitive beliefs that maintain the Cognitive-Attentional Syndrome (CAS), a pattern of perseverative negative thinking, threat monitoring, and unhelpful control strategies. The thesis reviews established psychotherapies including cognitive behavioural therapy (CBT), behavioral activation, and Mindfulness-Based Cognitive Therapy (MBCT), then focuses on MCT as a process-first approach that directly targets metacognitive beliefs and attention control through structured techniques such as the Attention Training Technique and detached mindfulness. Randomized controlled trials demonstrate large symptom reductions with MCT and promising maintenance of gains, though most studies have relied on waitlist controls rather than active comparators. The thesis concludes that MCT represents a coherent, brief, mechanism-aligned treatment option that can be paired with other interventions, and recommends future research prioritize larger, pre-registered, active-comparator trials with long-term follow-up and process-level endpoints to establish MCT's comparative effectiveness. Overall, this work underscores the importance of targeting both cognitive inefficiency and metacognitive regulation in depression care.