Classical group of myeloproliferative diseases is made of chronic myeloid leukemia (CML), polycythemia vera (PV), essential thrombocytosis (ET) and primary myelofibrosis (PMF). These are clonal disorders of hematopoietic progenitor cells that result in formation and accumulation of myeloid cells. They show a significant level of overlap in clinical and laboratory findings, especially at the beginning of the disease. CML arises from pluripotent cells and is responsible for 15 to 20 percent of leukemia in the adult population. BCR / ABL1 translocation is a specific genetic mark of chronic myeloid leukemia. Tyrosine kinase inhibitors (TKI) revolutionized the therapeutic approach to CML. In polycythemia vera (PV) increased mass of red blood cells and increased hematocrit are dominant, which together are responsible for all clinical manifestations of PV. Reduced serum erythropoietin (EPO) below normal values and bone-marrow analysis are important diagnostic criteria. Venipuncture is used as initial therapy to reduce the number of red cells. Essential thrombocytosis is neither a cytogenetic or morphologically defined entity. Peripheral blood smear reveals unexpectedly large platelets with impaired morphology and modes of accumulation followed by mild leukocytosis. The main cause of morbidity and mortality in patients with essential thrombocytosis is bleeding and thrombosis. Fibrosis in primary myelofibrosis suppresses hematopoiesis in bone marrow and leads to peripheral blood cytopenia with extensive extramedullary neoplastichematopoiesis. Despite the long history of research, the etiology of these diseases remain unknown, and so do possible ways of their prevention.