The cytological features of primary skin neoplasia are discussed in this chapter. Skin metastases originating from internal malignancies are not included in this chapter, as they will be covered in Chap. 9 . With rare exceptions, such as ulcerative squamous cell carcinoma, especially observed in cats, and exfoliative erythematous dermatitis in some cases of canine epitheliotropic lymphoma, nodules or plaques represent almost all skin neoplasia; for this reason, an approach based on clinical signs, as used for inflammatory skin diseases, could be not useful. Therefore, to discuss the cytology of skin neoplasms, a classification based on the cytological and histological morphology of the neoplastic cells has been used.