The porphyrins are an important class of naturally occurring macrocyclic compounds that play a very important role in the metabolism of living organisms. They have a universal biological distribution and were involved in the oldest metabolic phenomena on earth. Without porphyrins and their relative compounds, life as we know it would be impossible and therefore the knowledge of these systems and their excited states is essential in understanding a wide variety of biological processes, including oxygen binding, electron transfer, catalysis, and the initial photochemical step in photosynthesis. The objective of this volume is to present a full and critical spectral data coverage of the entire major fields relating to porphyrins, their precursors, catabolic derivatives, and related systems electronically.