Proton-coupled electron transfer (PCET) is a central principle for catalytic hydrogen and CO2 transformations. In this seminar, I will discuss how internal factors of metal–ligand identity, coordination geometry, and spin states interact with external influences such as pKa of proton donors and applied potentials to shape PCET pathways. Our studies with Co complexes show that spin interactions critically govern electron transfer, while investigations of Fe complexes reveal how maintaining the low spin states enables rapid, efficient multi-electron processes. Beyond the electronic framework, a proton itself can serve as an indispensable promoter of electron transfer. In W complexes, hydrogen bonding facilitates electron transfer, ultimately generating highly basic species capable of activating and reducing CO2. Together, these insights highlight how managing spin interactions, orbital reorganization, and proton coupling provides in-depth understanding of reaction mechanisms and design strategies for new catalytic systems.