A discussion of biochemical lesions in porphyria is complicated by the facts that (1) a number of different classifications of the human porphyrias have been proposed by different authorities,1 (2) certain biochemical abnormalities in the porphyrias can be considered secondary, in contrast to the fundamental enzymatic lesions, (3) a considerable amount of biochemical information is derived from studies of experimental porphyria, induced in several species by a variety of compounds, and (4) the fundamental biochemical lesions can be discussed from the standopint of metabolic pathways or biochemical genetics. The present discussion will briefly review the biochemical aspects of the porphyrias in light of the above four considerations. Theories about the enzyme defects in the porphyrias will also be discussed. Since such theories must ultimately explain both the chemical abnormalities and clinical aspects of the disease, it will be necessary first to review the chemistry of porphyrin biosynthesis and then