Redox reactions are central to the major element cycling, many cell cycles, many chemisorption and physisorption processes, trace element mobility from rocks and sediments toward wells, aquifers, trace element toxicity toward life forms, and most remediation schemes including water treatments; over the last three decades, the field has attracted a lot of scientists, and a great deal of researches has been done in redox chemistry. This book provides a very broad overview of the state of the art of understanding redox processes, which starts with giving a concise introduction that describes the origin, historical background, and the development of the redox definitions. The book is organized into two sections that include ten chapters and introduces, in Section 1, generalized electron balance theory and its applications in electrolytic redox systems, redox-active molecules and its applications in device memory, fundamentals and applications of flow batteries and their integration into antidirect current, and donor acceptor titrations of displacement and electronic transference. Section 2 introduces redox in biological processes, including roles of reactive oxygen species in respiration, metabolism, and regulations, and redox in physiological processes as redox-sensitive TRP channels TRPA1 and TRPM2. All chapters are written by different authors (with the exception of Chapter 1 [Introduction]). This clearly reflects the broad range of topics that have been covered by experts in the field.