Hemangiosarcoma (HSA) is malignant neoplasm characterized by aggressive growth, early metastasis and unfavorable prognosis. The tumor most commonly occurs in the spleen and heart in older dogs, most commonly affecting breeds like German Shepherd Dog, Golden Retriever and Labrador Retriever. Etiology and cellular origin of hemangiosarcoma has not yet been fully clarified. According to some research, HSA originates from multipotent progenitor cells of hematopoietic origin that can differentiate into three different molecular subtypes of HSA. For the tumor growth and development, the relationship between malignant cells and their environment is also essential. Dogs with hemangiosarcoma often show no clinical signs, and when clinical signs appear, they are associated with coagulopathy, cardiac arrhythmia or spontaneous rupture of HSA. Tumors can be found as single, multifocal or mixed red-purple nodules. Cross section of the tumor shows delicate, hemorrhagic neoplastic tissue while the solid areas are firm and white in color. Histologic types of hemangiosarcoma are capillary type, cavernous type and solid type. HSA cells are often spindle shaped with a round, oval or pleomorphic nucleus, prominent cell nucleolus and cytoplasm that is abundant, basophilic and vacuolated. Haematological and biochemical blood tests, coagulation blood test, radiography and ultrasound of thorax and abdomen are useful in diagnosis of the HSA, but the final diagnosis has to be confirmed by histological and immunohistological examination. The first choice in treating hemangiosarcoma is to use doxorubricin-based chemotherapy, with or without the addition of vincristine and cyclophosphamide. It is possible to remove the tumor surgically by excision, but with surgery itself, without the use of chemotherapy, the median survival time of dogs is no longer than 2-3 months, while with the use of chemotherapy the median survival time is 5-6 months. In addition to surgery and chemotherapy, new methods of treatment have recently been developed: inhalation chemotherapy, the administration of sterile stabilized liposomes, and the intra-abdominal chemotherapy application. Changing the diet in dogs with HSA is also recommended.