Mucoceles are pseudocystic, expansive, slow-growing nonneoplastic formations that mainly affect the frontal or ethmoidal paranasal sinuses. Mucoceles are caused by the overaccumulation of mucus, often due to sinus obstruction. They can cause the sinus to expand, thereby causing bone resorption. In cases of chronic infection, mucoceles can contain purulent secretions; these are termed “chronic pyocoele,” which should not be confused with acute infection. On occasion, mucoceles filled with pus can rupture, leading to associated osteomyelitis or the spread of infection. Because mucoceles can have many negative consequences, their early detection is quite important. Mucoceles are most often found in adults in their 30s or 40s; however, they have been reported in people of all ages, including children. There is no gender preference for mucoceles. The etiology of mucoceles can be divided into obstructive causes (e.g., polyps, trauma, surgery, and tumor compression), inflammatory causes (e.g., infection, cystic degeneration, and increased mucus secretion), and allergic causes. Histopathologically, mucoceles appear as mucus-containing cystic lesions that are covered by pseudostratified columnar respiratory epithelial tissue; in addition, they can contain chronic inflammatory cell infiltrate. Several factors have been identified that can predispose one to mucoceles, including previous paranasal sinus surgery (the most common predisposing factor), history of facial trauma, fibrosis, inflammation, and having paranasal sinus polyposis or a mass effect (e.g., malignancy, osteoma, fibrous dysplasia, Paget’s disease). In this chapter, paranasal sinus mucoceles are reviewed.