Histiocytic proliferative disorders are the source of some confusion, given their complex nomenclature and nonspecific clinical presentations. These diseases occur in both dogs and cats and derive from Langerhans cells, dendritic cells and macrophages. This presentation provides an overview of histiocytic diseases in dogs (histiocytoma, cutaneous Langerhans cell histiocytosis, cutaneous histiocytosis, systemic histiocytosis, histiocytic sarcoma and dendritic cell leukaemia) and in cats (progressive histiocytosis, histiocytic sarcoma and pulmonary Langerhans cell histiocytosis), including their cellular origins and clinical presentation. An overview of their diagnosis, including the use of cytology, histopathology and immunohistochemistry is also provided. This session is aimed at veterinary surgeons, veterinary students and veterinarians with an interest in pathology.