Publisher Summary The discovery of nickel(III) and nickel(I) in methanogenic bacteria and in other biological systems has focused attention on the redox chemistry of nickel. This chapter discusses the redox chemistry of nickel basing on the most recent advances. This chapter presents a discussion on the structural and electronic requirements of the four oxidation states—I, II, III, and IV— and of the primary physical techniques and probes used. A description of the most commonly used ligand types follows, together with an indication of recent trends in the area. Discussions conclude with a review of the recent mechanistic chemistry of the oxidation states in turn. The nickel(II) ion has a d8 electronic configuration and, with weak-field ligands such as H2O, it forms a six-coordinate ion with approximately octahedral symmetry and a paramagnetic (two unpaired electrons) 3A2 ground state. The X-ray crystal structures of a variety of high- and low-oxidation-state nickel complexes are now known. The technique most widely applied as a structural probe in the chemistry of both nickel(III) and nickel(I) is electron paramagnetic resonance. The mechanistic chemistry of nickel(IV) is dominated by substitution inert complexes and outer-sphere electron transfer reactions.