This article provides a comprehensive review of proton-coupled electron transfer (PCET), a fundamental process in chemistry and biology. PCET involves the simultaneous transfer of both electrons and protons, which occurs in various biological processes and chemical reactions. The review covers the historical context, mechanisms, and theoretical foundations of PCET, emphasizing its importance in energy conversion and storage. Key aspects discussed include: 1. **Introduction to PCET**: The term PCET was introduced in 1981 to describe the concerted transfer of electrons and protons, exemplified by the comproportionation reaction between RuIV and RuII. PCET is crucial in processes like water splitting and photosynthesis. 2. **Concerted Electron-Proton Transfer (EPT)**: EPT involves the simultaneous transfer of electrons and protons, occurring through a single concerted step. The review discusses the mechanisms of EPT, including pre-association, proton tunneling, and the role of hydrogen bonding. 3. **MS-EPT (Multiple Site Electron-Proton Transfer)**: This involves the transfer of electrons and protons to different acceptors or donors, often observed in biological systems. The review highlights the complexity and diversity of MS-EPT pathways and their energetic advantages. 4. **Theoretical Foundations**: The article delves into the theoretical aspects of EPT, including the pre-association step, proton and electron transfer mechanisms, and the role of vibrational modes. It also discusses the impact of pH on EPT dynamics and the thermodynamics of PCET reactions. 5. **Thermodynamics and Structure**: The review explores how changes in electron content affect acid-base properties, leading to dramatic changes in pKa values. It also examines the pH dependence of PCET half-reactions and the role of ligands in PCET processes. The article concludes by emphasizing the significance of PCET in understanding and designing efficient energy conversion and storage systems in both chemistry and biology.This article provides a comprehensive review of proton-coupled electron transfer (PCET), a fundamental process in chemistry and biology. PCET involves the simultaneous transfer of both electrons and protons, which occurs in various biological processes and chemical reactions. The review covers the historical context, mechanisms, and theoretical foundations of PCET, emphasizing its importance in energy conversion and storage. Key aspects discussed include: 1. **Introduction to PCET**: The term PCET was introduced in 1981 to describe the concerted transfer of electrons and protons, exemplified by the comproportionation reaction between RuIV and RuII. PCET is crucial in processes like water splitting and photosynthesis. 2. **Concerted Electron-Proton Transfer (EPT)**: EPT involves the simultaneous transfer of electrons and protons, occurring through a single concerted step. The review discusses the mechanisms of EPT, including pre-association, proton tunneling, and the role of hydrogen bonding. 3. **MS-EPT (Multiple Site Electron-Proton Transfer)**: This involves the transfer of electrons and protons to different acceptors or donors, often observed in biological systems. The review highlights the complexity and diversity of MS-EPT pathways and their energetic advantages. 4. **Theoretical Foundations**: The article delves into the theoretical aspects of EPT, including the pre-association step, proton and electron transfer mechanisms, and the role of vibrational modes. It also discusses the impact of pH on EPT dynamics and the thermodynamics of PCET reactions. 5. **Thermodynamics and Structure**: The review explores how changes in electron content affect acid-base properties, leading to dramatic changes in pKa values. It also examines the pH dependence of PCET half-reactions and the role of ligands in PCET processes. The article concludes by emphasizing the significance of PCET in understanding and designing efficient energy conversion and storage systems in both chemistry and biology.