The merging of photoredox and nickel catalysis has revolutionized the field of C–C cross-coupling. However, in comparison to the development of synthetic methods, detailed mechanistic investigations of these catalytic systems are lagging. In this vein, computational tools have been applied to elucidate the mechanistic pictures, allowing for the rational design of new catalysts and the development of novel reactivity. Based on the reported studies, it appears that the mechanistic picture of catalytic systems is not generally applicable, but is rather dependent on the specific choice of substrate, ligands, photocatalysts, etc. Therefore, the challenges and opportunities of investigating the mechanisms of Ni/photoredox dual-catalyzed C–C cross-coupling reactions were first discussed (Chapter 1). Using both quantum mechanics and molecular dynamics simulations, the mechanism of the tertiary radical cross-coupling between alkyl trifluoroborates and aryl bromides was then investigated, revealing the effect of ligands and the properties of alkyl radicals (Chapter 2). After exploring the mechanism of two-component Ni/photoredox dual-catalyzed C(sp2)–C(sp3) cross-couplings, further mechanistic investigation was conducted for multicomponent cross-coupling reactions, revealing the factors controlling reactivity and selectivity in these complex catalytic transformations. In a multicomponent C–H activation/cross-coupling reaction, the origin of chemoselectivity between two-component versus three-component products was studied, showcasing the effect of intramolecular H-bonding (Chapter 3). Moreover, the mechanism of a novel enantioselective olefin difunctionalization was computationally investigated, identifying the radical addition step as the enantioselectivity-determining step (Chapter 4). Expanded to a wider scope of catalytic systems and reagents, the catalytic cycles of an enantioselective dicarbofunctionalization of vinylphosphonate were explored, demonstrating the origin of stereo- and enantioselectivity of this transformation (Chapter 5). Given the complexity of the mechanistic pictures of these nickel metallaphotoredox systems, the need for more accurate computational methods, readily available and user-friendly dynamics simulation tools, and data-driven approaches is clear in order to understand at the molecular level of these transformations.