<p>Cancer immunotherapy has revolutionised oncology by utilising immune-mediated mechanisms to achieve durable anti-tumour responses and long-term clinical benefits. Unlike traditional cytotoxic therapies, immunotherapeutic approaches modulate host-tumour interactions and restore immune surveillance, thereby enabling sustainable tumour control. This review provides a comprehensive synthesis covering the biological foundations and clinical development of cancer immunotherapy, cancer-immunity system interactions, tumour immunoediting, and the cancer-immunity cycle as a conceptual framework for therapeutic intervention. Major immunotherapeutic approaches, including immune checkpoint inhibitors, adoptive cell therapies, cancer vaccines, cytokine-based therapies, and oncolytic viruses, have been systematically reviewed for their mechanisms of action and clinical applications. Particular emphasis is placed on regulatory agency-approved agents, indication-based clinical use, predictive biomarkers, and rational combination strategies. Furthermore, this review addresses key challenges that limit therapeutic efficacy, including primary and acquired resistance, tumour heterogeneity, immune escape mechanisms, and the immunosuppressive tumour microenvironment. New directions such as personalised immunotherapy, neoantigen-based targeting, microbiome-immune interactions, and artificial intelligence-supported systems immunology are also discussed. By integrating mechanistic insights with clinical evidence and resistance frameworks, this review provides a consolidated perspective on the current state and future trajectory of cancer immunotherapy, offering important implications for translational research and clinical practice.</p>