This book is designed for Electrochemists and Biologists interested in Marcus Electron Transfer theory, particularly those who are experimentalists with little background in theoretical Chemical Kinetics, Statistical Mechanics, or Semiclassical Mechanics. It offers a beginner-friendly approach to this complex field.The book features excerpts from Marcus' original papers, complemented by explanatory notes and comments from the author to make the theory more accessible. It provides practical insights into electron transfer mechanisms, focusing on their applications in both electrochemical systems and biological contexts. Key topics such as electron and proton transfer, potential energy surfaces, and the inverted free energy effect are explained clearly, with step-by-step guidance and real-world examples.