Porphyrin and its derivatives have been widely studied due to their numerous physical/chemical properties and their applications in fields such as catalysis, sensors and photodynamic cancer therapy. In this book, the authors present current research on porphyrins including the use of porphyrin autofluorescence for cancer diagnosis; application of porphyrins for neoangiogenic disorders; electrocatalytic reduction of oxygen by cobalt porphyrins for fuel cell applications; construction and supramolecular chemistry of porphyrin nanorings; reactivity of porphyrin radical cations and dications towards nucleophiles; and the synthesis of chlorins and bacteriochloris.