Appendiceal mucinous neoplasms are rare tumours whose terminology, staging and management remain difficult to align with conventional colorectal cancer paradigms. Low-grade appendiceal mucinous neoplasm (LAMN) is well established, whereas high-grade appendiceal mucinous neoplasm (HAMN) remains less clearly defined in clinical practice. This review summarises current evidence on the classification, pathological distinction, clinical behaviour, prognosis and management of LAMN and HAMN. Both entities share a non-infiltrative growth pattern, but HAMN is distinguished by high-grade cytologic atypia and a greater association with high-grade peritoneal disease once dissemination occurs. In appendix-confined, non- perforated disease, HAMN seems to behave more like LAMN than invasive adenocarcinoma, so high-grade cytology alone should not be taken as an indication for routine right hemicolectomy. When perforation, extra-appendiceal mucin or peritoneal involvement is present, management should be escalated to specialist multidisciplinary assessment, with surveillance and consideration of cytoreductive surgery with hyperthermic intraperitoneal chemotherapy tailored to disease extent. Management should therefore integrate histology with disease extent, perforation status, peritoneal cellularity and dissemination pattern rather than rely on the cytologic grade alone.