In the first-row of d-block metals, ten elements are included, such as scandium (Sc, 3d1), titanium (Ti, 3d2), vanadium (V, 3d3), chromium (Cr, 3d54s1), manganese (Mn, 3d5), iron (Fe, 3d6), cobalt (Co, 3d7), nickel (Ni, 3d8), copper (Cu, 3d104s1) and zinc (Zn, 3d10). The synthesis, characterization, and reactivity of first-row d-block metal-superoxo complexes are discussed together with the structures of the end-on (η1) and side-on (η2) metal-superoxo complexes in this review article. Electron transfer from electron donors to O2 is enhanced by binding of Sc3+ to produce an end-on type Sc(iii)-superoxo complex. Metal-superoxo complexes such as Ti(iv)-superoxo, oxovanadium(v)-superoxo, Cr(iii)-superoxo, Fe(iii)-superoxo, Co(iii)-superoxo, Ni(iii)-superoxo and Cu(ii)-superoxo species generally undergo hydrogen atom transfer reactions. A Cr(iii)-superoxo complex undergoes not only hydrogen atom transfer but also oxygen atom transfer reactions. In the presence of protons (e.g., trifluoromethanesulfonic acid, HOTf), much enhanced acid catalysis was observed in oxygen atom transfer reactions from a nonheme Cr(iii)-superoxo complex, [(Cl)(TMC)CrIII(O2)]+, to thioanisole. The enhanced reactivity of [(Cl)(TMC)CrIII(O2)]+ by HOTf results from proton-coupled electron transfer (PCET) from electron donors, including thioanisole, to [(Cl)(TMC)CrIII(O2)]+. A manganese(iv)-superoxo complex plays a very important role in thermal and photoinduced dioxygen activation by a Mn(iii) corrolazine complex. A metal-superoxide complex using the last element in the first-row of transition metals, that is a Zn(ii)-superoxide complex, is produced to accelerate the reduction of O2- in a SOD (superoxide dismutase) model.