The rational design of well-defined, first row transition metal complexes that can activate dioxygen has been a challenging goal for the synthetic inorganic Chemist. The activation of O-2 is important in part because of its central role in the functioning of metalloenzymes, which utilize O-2 to perform a number: of challenging reactions including the highly selective oxidation of various substrates. There is also great interest in utilizing O-2, an abundant and environmentally benign oxidant, in synthetic catalytic oxidation systems. This Perspective brings together recent examples of biomimetic Fe and Mn complexes that can activate O-2 in heme Or nonheme-type ligand environments. The use of oxidants such as hypervalent iodine (e.g, ArIO), peracids (e.g., m-CPBA), peroxides (e.g., H2O2) or even superoxide is a popular choice for accessing well-characterized metal-superoxo, metal peroxo, or metal-oxo :species, but:the instances of biomimetic Fe/Mn complexes that react with dioxygen to yield such obserable metal-oxygen species are surprisingly few. This Perspective focuses on mononuclear Fe and Mn complexes that exhibit reactivity with O-2 and lead to spectroscopically observablemetal-oxygen species, and/or oxidize biologically relevant substrates-Analysis of these examples reveals that solvent, spin state, redox potential, external co-reductants, and ligand architecture can all play important roles in the O-2 activation process.