Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired clonal hematopoietic disorder characterized by the existence of somatic mutations in the PIG-A (phosphatidylinositolglycan complementation class A) gene, which encodes for a protein involved in the biosynthesis of the glycosyl phosphatidylinositol (GPI) molecule that serves as an anchor for many cell surface proteins. This genetic alteration translates into a total or partial deficiency in the PNH clone of surface proteins attached to the cell by a GPI anchor. Evaluation of deficient expression of GPI-associated proteins is currently used for the diagnosis of paroxysmal nocturnal hemoglobinuria. Among other proteins, deficiency of CD55 and CD59 leads to an increased susceptibility of PNH cells to complement-mediated cell lysis. Variable degrees of cytopenia, an increased susceptibility to infections and recurrent thrombotic events are other symptoms of the disease. In this paper we review the recent advances in the knowledge about the pathogenic mechanisms of the disease and the current approaches for the diagnosis and monitoring of PNH patients.