The porphyrias are a group of disorders caused by defects in the biosynthesis of haem. Their relevance to the skin arises from the phototoxic properties of the porphyrins, which accumulate in most porphyrias and cause photosensitivity. The majority of the porphyrias are inherited. Many of them affect other organs as well as the skin. The recognition and management of both the genetic and internal consequences of porphyrias presenting in the skin are a key challenge for the dermatologist. Clinical management in these disorders is made easier when the clinician understands their theoretical basis. Thus, this section is divided into two halves. The first provides a theoretical basis for understanding the porphyrias, the general principles of clinical management and a clinician's guide to laboratory testing. The second half covers individual porphyries in detail.

Keywords porphyria, porphyrin, porphyria cutanea tarda, protoporphyria, variegate, Günther disease, erythropoietic, pseudoporphyria