The activation of dioxygen in biological systems has long attracted the interest of chemists and biochemists because of the metabolic importance of the substrates and products of such reactions. The efficiency and specificity of the enzymes that activate dioxygen have raised important mechanistic questions as to the nature of the chemical species that carry out such reactions. The nonheme iron enzymes represent the most diverse subset of enzymes associated with oxygen activation, in terms of both the nature of the active site and the types of oxidations that are carried out. Since the publication of the first edition of this book, there have been many significant advances in the nonheme iron field; in particular, the number of crystal structures of nonheme iron enzymes has tripled. This chapter will summarize highlights of the many enzymes that fall in this category. For greater depth, the reader is directed to a number of reviews that have recently appeared [1-5].