Mucinous epithelial neoplasia of the appendix accounts for 38% of all appendix tumors. They have been described under a variety of names, and their classification has long been confused and imprecise. An effort to classify them has been underway for some years now. It is now accepted that these lesions form a continuous spectrum ranging from low-grade, non-invasive neoplasia to high-grade, invasive neoplasia. Low-grade mucinous neoplasia is the most common epithelial neoplastic lesion of the appendix. Morphologically, they resemble benign lesions. Their complications, and their responsibility for the syndrome known as pseudomyxoma peritonei, can pose real management problems.