Hairy cell leukemia (HCL) is a chronic B-cell lymphoproliferative disorder characterized by pancytopenia and variable infiltration of the reticuloendothelial system with "hairy" lymphocytes. HCL is more common in men than women and has a median age of diagnosis of 52 yr. Typically, patients with HCL respond well to purine analog-based therapy. The purpose of this review will be to establish the current status of HCL with respect to its pathophysiology, diagnosis, management, and future directions.