This introduction presents an overview of key concepts discussed in this book. The book deals with organic π‐electron systems and related organo main group compounds that show such reversible one‐electron transfer. Redox potential is the important measure for redox systems, by which one predicts how easily one‐electron oxidation or reduction takes place with other redox reagents. For the measurement of redox potential, cyclic voltammetry is the first choice, because not only the redox potential but also the stability of the species generated after electron transfer can be observed. Most organic redox compounds do not dissolve in water, and hence their electrochemical measurements have to be taken in nonaqueous solvents such as dichloromethane, DMF, and acetonitrile. The emerging interest in organic materials advances the development of organic redox systems and deepens the knowledge of their various properties such as electronic state and bonding interaction between stable radicals, magnetism, and conductivity.