Mast cell tumours are one of the most common type of skin tumours in dogs. They arise from mast cells, which are special types of cell which play a part in the animal’s immune system. They are primarily found in the skin and play a part in inflammatory reactions, releasing a chemical called histamine. Mast cell tumours can vary greatly in shape and appearance (which often makes diagnosis difficult) and they can be present on or under the skin (in the subcutaneous tissue). In rare cases, mast cell tumours can arise from areas other than the skin. Mast cell tumours can occur in all ages and breeds of dogs, but the average age at presentation is ~8-9 years of age. The exact cause of mast cell tumours is still uncertain. A viral source has been suggested, as well as hereditary and environmental factors. It is quite possible that there are a variety of different causes for the development of this tumour.