Canine cutaneous histiocytomas originate from Langerhans cells. Multiple histiocytomas are referred to as cutaneous Langerhans cell histiocytosis. Feline pulmonary Langerhans cell histiocytosis causes respiratory failure owing to extensive lung infiltration. Localized and disseminated histiocytic sarcomas usually arise from interstitial dendritic cells. Primary sites include spleen, lung, skin, brain (meninges), lymph node, bone marrow, and synovial tissues of limbs. An initially indolent form of localized histiocytic sarcomas, progressive histiocytosis, originates in the skin of cats. Hemophagocytic histiocytic sarcomas originates in splenic red pulp macrophages. Canine reactive histiocytoses (systemic histiocytosis and cutaneous histiocytosis) are complex inflammatory diseases with underlying immune dysregulation.