Our understanding of the classification, function, mechanism, and structure of the enzymes which incorporate atoms of oxygen from atmospheric molecular oxygen during catalysis is based on the thoughtful and technically challenging experiments of two giants in the field of Biochemistry, Howard Mason and Osamu Hayaishi. This volume celebrates the 50th anniversary of the discovery and characterization of these "oxygenase" enzymes and provides a broad view of how far this area of research has advanced. Professor Hayaishi describes herein his perspective on the background and major discoveries which led to the development of this field. Regrettably Howard Mason passed away at age 88 in 2003. I am indeed fortunate to have been a Ph.D. student with Howard and to have the opportunity to briefly review his role in the development of this field for this special commemorative issue of BBRC.