An Historical Introduction to Porphyrin and Chlorophyll Synthesis -- Biosynthesis of 5-Aminolevulinic Acid -- 5-Aminolaevulinic Acid Dehydratase, Porphobilinogen Deaminase and Uroporphyrinogen III Synthase -- Transformation of Uroporphyrinogen III into Protohaem -- Inherited Disorders of Haem Synthesis: -- Heme Degradation: -- Regulation of Mammalian Heme Biosynthesis -- Tetrapyrroles in Photodynamic Therapy -- Heme Transport and Incorporation into Proteins -- Heme and Hemoproteins -- Novel Heme-Protein Interactions- Some More Radical Than Others -- Synthesis and Role of Bilins in Photosynthetic Organisms -- Phytochromes: -- Biosynthesis of Chlorophyll and Barteriochlorophyll -- Regulation of Tetrapyrrole Synthesis in Higher Plants -- Regulation of the Late Steps of Chlorophyll Biosynthesis -- Chlorophyll Breakdown -- Vitamin B12: Biosynthesis of the Corrin Ring -- Conversion of Cobinamide into Coenzyme B12 -- The Regulation of Cobalamin Biosynthesis -- Coenzyme B12-Catalyzed Radical Isomerizations -- Biosynthesis of Siroheme and Coenzyme F430 -- Role of Coenzyme F430 in Methanogenesis -- The Role of Siroheme in Sulfite and Nitrite Reductases -- The Role of Heme d 1 in Denitrification. Excluding the biological polymers proteins, lipids and nucleic acids, modified tetrapyrroles are the biological molecules that have had the greatest impact on the evolution of life over the past 4 billion years. They are involved in a wide variety of fundamental processes that underpin central primary metabolism in all kingdoms of life, from photosynthesis to methanogenesis. Moreover, they bring colour into the world and it is for this reason that these compounds have been appropriately dubbed the 'pigments of life'. To understand how and why these molecules have been so universally integrated into the life processes one has to appreciate the chemical properties of the tetrapyrrole scaffold and, where appropriate, the chemical characteristics of the centrally chelated metal ion. This book addresses why these molecules are employed in Nature, how they are made and what happens to them after they have finished their usefulness.