Many members of the superfamily of hemeproteins, known as cytochrome P450 (P450 or CYP), are currently described in the literature (over 2000 at the date of this writing) [see Nelson, 2003 (http://drnelson.utmem.edu/CytochromeP450.html)]. In mammalian tissues, the P450s play central roles in drug and xenobiotic metabolism as well as steroid hormone synthesis, fat-soluble vitamin metabolism, and the conversion of polyunsaturated fatty acids to biologically active molecules. P450s also play a major role in plants by catalyzing the synthesis of a large number of secondary metabolites. Today we appreciate the unique oxygen chemistry catalyzed by the P450 enzymes as well as the dramatic effect of protein structural changes resulting in modifications of substrate specificity. Recent scientific advances have shown the importance of genetic differences (polymorphisms) in altering the physiological response of an animal to endo- and exo-biotic chemicals. In many instances these changes can be directly attributed to small differences in the amino acid sequence of a P450. The present article describes some of the early events associated with the establishment of the biological function of P450s. The 1950s and 1960s showed the transition of P450 from an unknown spectroscopic curiosity to the major player it now occupies in maintaining cellular homeostasis. The P450s are now recognized to occupy a great variety of phylogenetically distributed isoform activities. Much has been learned about the P450s, but much more remains as poorly understood. It has been almost 50 years since this class of unique proteins were discovered and their catalytic functions characterized. The present article describes the background and early history of research leading to our present knowledge of the cytochromes P450. Hopefully we will learn lessons from this history as we venture forward down the path of future scientific discovery.