Heme-containing proteins play an essential part in the physiological transport of dioxygen, and in the oxidative metabolism of both endogenous and exogenous substrates. These latter processes occur through a series of highly reactive heme–oxygen intermediates. The development of synthetic analogues of these proteins and metal–oxygen intermediates has helped to elucidate the molecular mechanisms of these proteins and to establish the fundamental criteria for metal binding and activation of O2. This chapter outlines the basic chemical principles that govern the binding and activation of dioxygen by metalloporphyrinoid centers. An overview of the structures and mechanisms of heme mono- and dioxygenases is provided, with an emphasis on the factors that stabilize or activate the heme/O2 interactions. Focus is given to iron and manganese porphyrinoid complexes, which include porphyrins, corroles, corrolazines, porphyrazines, and phthalocyanines. Recent examples of metal/O2 species are discussed, together with catalytic, O2-dependent oxidations of different substrates mediated by Mn/Fe porphyrinoid complexes.