Histiocytic sarcoma (HS) and hemangiosarcoma (HSA) are uncommon and aggressive neoplasms that develop much more frequently in dogs than in cats. Breed-specific predispositions have been identified for both cancers. The development of novel diagnostics is underway and may aid in earlier diagnosis. Therapeutic approaches to HS and HSA depend on the stage of disease and may include surgery, radiation therapy, and chemotherapy. Such interventions improve outcome; however, aside from a small number of clinical circumstances, both diseases are considered largely incurable. Continued efforts toward the identification of driver mutations and subsequent druggable targets may lead to improvements in long-term prognosis.