Porphyrias are a group of metabolic disorders in which there are defects in the normal pathway for the biosynthesis of heme, the critical prosthetic group for numerous hemoproteins. The clinical manifestations of the porphyrias can be highly varied, and patients may present to general physicians and be referred to a wide variety of subspecialists because of these manifestations. However, two major clinical forms are represented by the so-called "acute" porphyrias, in which patients suffer recurrent bouts of pain, especially pain in the abdomen, and the "cutaneous" porphyrias, in which patients have painful skin lesions. Knowledge of the factors chiefly responsible for regulating the rate of synthesis of heme has helped to explain how drugs and other factors may cause porphyria. Knowledge of the physical and chemical properties of porphyrins also forms an important part of the foundation for understanding the clinical manifestations of these diseases. Thus, the porphyrias can best be understood after reviewing the chemical properties of porphyrins and heme and the control of their biosynthesis.