Erythrocytosis, characterized by the overproduction of red blood cells, can be classified into relative and absolute types, with the latter further divided into primary (polycythemia vera) and secondary causes. The document outlines the epidemiology, clinical features, diagnostic criteria, and treatment options for polycythemia vera, a condition often diagnosed in older males and associated with genetic mutations like JAK2 V617F. Without treatment, patients face severe complications and a reduced median survival, whereas recent therapies have improved longevity significantly.