This chapter is conceived as the gateway to more specific sections in the book. Its main aim is to introduce all the reactions of interest for fully understanding further development and applications of the EF process. The 50 reactions provided condense all the phenomena occurring in such a complex system and serve as the platform to justify the need of different devices and setups when treating water matrices of very different nature. In addition, all the key operation parameters for H2O2 electrogeneration and water decontamination are discussed. Subsections devoted to explaining the effect of the electrolyte composition, cell design, cathode and anode nature, catalyst source, hydrodynamic conditions, solution pH, and operation mode (potentiostatic or galvanostatic) are set out in summarized form, in order to present all the crucial information without intending to duplicate ideas that will be already given in subsequent chapters.