Metal-oxyl (Mn+-O-center dot) complexes having an oxyl radical ligand, which are electronically equivalent to well-known metal-oxo (m((n+1)+)=O) complexes, are surveyed as a new category of metal-based oxidants. Detection and characterization of Mn+-O-center dot species have been made in some cases, although proposals and characterization of the species are mostly done on the basis of density functional theory (DFT) calculations. The reactivity of Mn+-O-center dot complexes will provide a way to achieve potentially difficult oxidative conversion of substrates. This Viewpoint will provide state-of-the-art knowledge on the Mn+-O-center dot species in terms of the formation, characterization, and DFT-based proposals to shed light on the characteristics of the intriguing oxidatively active species.