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Anal sacs are present in most carnivores. They are a pair of bilaterally symmetrical structures (4–15 mm in size) located lateral to the anus, between the external and internal anal sphincter muscles. The anal sacs are two skin invaginations connected to the surface of the anocutaneous junction by short ducts. The wall of the anla sacs is lined by an often pigmented squamous cornifying epithelium, surrounded by connective tissue in which a large number of epitrichial (apocrine) and sebaceous glands are embedded. Secretions from the anal sacs are a combination of these glandular products and desquamated corneocytes. Secretion is released during defecation. The function of the anal sac secretion is unknown, but owing to its typical odour, an olfactory role in social and territorial identification is likely. The odour is due to the products of glandular secretion breakdown, such as trimethylamine, butyric acid, propionic acid, indoles and scatoles. No difference in odour is reported between intact and neutered dogs. This chapter looks at Differential diagnosis; Clinical approach; and Treatment. Differential diagnoses are also considered in highlighted boxes.