Book excerpt: Scientists in such fields as mathematics, physics, chemistry, biochemistry, biology, and medicine are currently involved in investigations of porphyrins and their numerous analogues and derivatives. Porphyrins are being used as platforms for the study of theoretical principles, as catalysts, as drugs, as electronic devices, and as spectroscopic probes in biology and medicine. The need for an up-to-date and authoritative treatise on the porphyrin system has met with universal acclaim amongst scientists and investigators.