This review summarizes the mechanisms of photoredox catalysis involving Ni-bipyridine (bpy) complexes, focusing on both photosensitized and direct excitation processes. Photoredox catalysis enables the transformation of organic reactants into desired products by converting photon energy into chemical potential to drive redox and bond transformation processes. Despite its importance in cross-coupling reactions and other transformations, the mechanistic details of these processes remain only superficially understood. The review highlights the role of the ground- and excited-state geometric and electronic structures of key Ni-based intermediates in defining photoredox reaction mechanisms, which in turn determine substrate scope. It also identifies knowledge gaps that motivate future mechanistic studies and the development of synergistic research approaches across physical, organic, and inorganic chemistry. The review discusses the electronic structure of Ni complexes, emphasizing the distinct electronic structures of Ni(II) intermediates and their implications for reactivity. Ni(II) is most stable in the 2+ oxidation state with a d⁸ electron configuration. The geometry and electronic structure of Ni-bpy complexes influence their light-harvesting ability and reactivity. The review also explores the mechanisms of photoredox catalysis, including Reductive SET, Oxidative SET, and Photosensitization for Homolysis, each with its own key considerations and experimental evidence. The Reductive SET mechanism involves the Ir(III) photosensitizer reducing Ni(I) to Ni(0), which then undergoes oxidative addition with an aryl halide. The Oxidative SET mechanism involves the Ir(III) photosensitizer oxidizing a Ni(II) complex to Ni(III), which then undergoes reductive elimination. The Photosensitization for Homolysis mechanism involves either SET or triplet energy transfer (³EnT) to initiate bond homolysis. The review also discusses the importance of the electronic structure of Ni complexes in determining the mechanistic pathway for catalysis. The electronic structure of Ni-bpy complexes influences their ability to absorb light and participate in redox reactions. The review highlights the role of ligand field theory in determining the electronic structure of Ni complexes and their reactivity. The review concludes that a deeper understanding of the electronic structure and reactivity of Ni complexes is essential for the development of more efficient and sustainable photoredox catalytic processes.This review summarizes the mechanisms of photoredox catalysis involving Ni-bipyridine (bpy) complexes, focusing on both photosensitized and direct excitation processes. Photoredox catalysis enables the transformation of organic reactants into desired products by converting photon energy into chemical potential to drive redox and bond transformation processes. Despite its importance in cross-coupling reactions and other transformations, the mechanistic details of these processes remain only superficially understood. The review highlights the role of the ground- and excited-state geometric and electronic structures of key Ni-based intermediates in defining photoredox reaction mechanisms, which in turn determine substrate scope. It also identifies knowledge gaps that motivate future mechanistic studies and the development of synergistic research approaches across physical, organic, and inorganic chemistry. The review discusses the electronic structure of Ni complexes, emphasizing the distinct electronic structures of Ni(II) intermediates and their implications for reactivity. Ni(II) is most stable in the 2+ oxidation state with a d⁸ electron configuration. The geometry and electronic structure of Ni-bpy complexes influence their light-harvesting ability and reactivity. The review also explores the mechanisms of photoredox catalysis, including Reductive SET, Oxidative SET, and Photosensitization for Homolysis, each with its own key considerations and experimental evidence. The Reductive SET mechanism involves the Ir(III) photosensitizer reducing Ni(I) to Ni(0), which then undergoes oxidative addition with an aryl halide. The Oxidative SET mechanism involves the Ir(III) photosensitizer oxidizing a Ni(II) complex to Ni(III), which then undergoes reductive elimination. The Photosensitization for Homolysis mechanism involves either SET or triplet energy transfer (³EnT) to initiate bond homolysis. The review also discusses the importance of the electronic structure of Ni complexes in determining the mechanistic pathway for catalysis. The electronic structure of Ni-bpy complexes influences their ability to absorb light and participate in redox reactions. The review highlights the role of ligand field theory in determining the electronic structure of Ni complexes and their reactivity. The review concludes that a deeper understanding of the electronic structure and reactivity of Ni complexes is essential for the development of more efficient and sustainable photoredox catalytic processes.