Heme oxygenase is rapidly taking its place as the centerpiece of multiple inter­ acting metabolic systems. Only 25 years ago heme oxygenase and its metabolic prod­ ucts appeared to be merely a simple metabolic system-one substrate, heme; one enzyme, heme oxygenase; and one set of products, iron to be recycled, and bilirubin and carbon monoxide to be disposed. From a group of about 25 people in 1974, as judged by attendance at various Gordon conferences, heme oxygenase has, in the year 2000, attracted working scientists-and clinicians I might add-by the hundreds and has produced referenced publications by the thousands. It is well-deserved attention. Heme oxygenase system is now similar to the metabolic networks surrounding glucose in those complex maps of glycolytic and non-glycolytic metabolic pathways, which we had to memorize as students. The relevance of heme oxygenase to regulatory biology was recognized many years ago, but the work conducted over the past five years has created a new wave of emphasis focusing on genetic manipulation to alter heme oxygenase gene expression, the regulatory actions of heme oxygenase products including carbon monoxide, and the significance of changes in the heme oxygenase system. The physiological and pathological relevance of heme oxygenase in the brain, heart, liver, bone marrow, organ transplant, lung and kidney, opens many areas of investigation in various disciplines. Advances in the pharmacology of bilirubin and its ability as an antioxidant have provided a new avenue in clinical research.