Hairy cell leukemia (HCL) is a B-cell chronic lymphoproliferative disorder that was initially described as leukemic reticuloendotheliosis. The disease is characterized by monocytopenia, organomegaly, constitutional symptoms, and bone marrow fibrosis. Significant advances have improved the diagnosis and management of HCL over the last 55 years. Although HCL has an indolent course, most patients will require treatment of the disease. Indications to initiate therapy include disease-related symptoms, signs of bone marrow failure, or frequent infections. Asymptomatic patients without cytopenias can be observed without the need for therapeutic interventions. Therapeutic options usually consist of chemotherapy, immunotherapy, biological agents, and surgery.