The aim of this paper was to describe the details of the pathophysiology of leukemias and lymphomas of several animal species, with emphasis on dogs and cats. The pathophysiology of leukemias and lymphomas in animals is not sufficiently elucidated and needs additional systematization and classification, with additional animal models and research in order to resolve many of today’s unknowns, from etiology to the mechanisms of the two diseases. The occurrence of hematological neoplasms is related to the cell cycle and its control and occurs due to errors in it. Precisely this, in combination with specific characteristics of tumorous cells, results in numerous mutations and uncontrolled divisions of once before healthy and functional cells. The main difference between leukemia and lymphoma is that leukemia occurs primarily in the bone marrow and blood affecting almost all cell lineage. Lymphoma affects only lymphocytes, and it occurs in certain organs. It is most often seen in organs which have the highest concentration of lymphocytes - those that form the basis of the immune response. Lymphoma, which originates from cells of the immune system - lymphocytes, can by its cellular origin be of T or B type, and in addition may differ according to its anatomical location, immunophenotype, histological structure and biological behavior. Clinical forms are numerous, and the incidence of these varies from species to species, where in dogs the most common variant is the multicentric form. In cats, it is a lymphoma of the gastrointestinal tract, and the onset of the disease is often associated with a positive FeLV test. The most important disease variant in farm animals is the enzootic bovine leukosis, which is caused by a retrovirus of the genus Deltaretrovirus and it affects cattle, sheep, and goats. The pathogenesis of enzootic bovine leukosis starts when viral DNA integrates into the DNA of the host cell by reverse transcriptase enzyme, and it primarily infects B cells and causes persistent lymphocytosis. The disease affects only adult bovine and most of them do not usually show clinical signs. This disease is controlled by the law according to the "Order on measures to protect animal health from infectious and parasitic diseases and their financing in 2021." 62 Leukemia is characterized by the proliferation of malignant hematopoietic cells, which originate from the bone marrow. It occurs in all domestic animals, and it is less frequent than lymphoma. It is classified into acute or chronic, and lymphoid (lymphocytic/lymphoblastic) or myeloid, by which we differentiate main types: acute lymphoblastic leukemia (ALL), acute myeloblastic leukemia (AML), chronic lymphocytic leukemia (CLL) and chronic myeloid leukemia (CML). Acute lymphoblastic leukemia shows a little of cell differentiation, and the dominant cell population are blasts and/or prolymphocytes. On the other hand, acute myeloblastic leukemia is the one where leukocytosis is present, with a number of circulating atypical blasts and cells are differentiated into granulocyte, monocyte, erythrocyte, and megakaryocyte cell lineage. In dogs, it has been reported a higher incidence of acute myeloblastic leukemia, whereas in cats acute lymphoblastic leukemia is more common, and it is often correlated with feline leukemia virus infection. Chronic leukemia can also be lymphoid or myeloid, and those have their own subtypes. There is not much known about it, and it occurs less frequently than the acute one, but studies show that it affects older dogs and cats. In cats, both types are extremely rare in the population, whereas in dogs, chronic lymphoid leukemia is somewhat more common than the one of myeloid origin. Chronic lymphoid leukemia manifests as persistent peripheral lymphocytosis where lymphocytes are morphologically mature and persist in the body for more than three months. Chronic myeloid leukemia (CML) includes several subtypes - chronic myeloid or granulocytic, chronic monocytic and chronic myelomonocytic leukemia, and it is clinically almost identical to chronic lymphocytic leukemia.