In this chapter the field of redox polymers and the main contributions of the following chapters are introduced. First, we present the definitions and classification of redox polymers. The type of redox polymer will depend on the location of the redox center, whether it is located in the polymer backbone or as a pendant group, on the conjugation of the polymer backbone and the chemical nature of the redox center. Next, the main characterization methods used for assessing the properties of redox polymers such as cyclic voltammetry or electrochemical impedance spectroscopy will be discussed. The main applications of redox polymers in energy will be discussed focusing mostly on the use of redox polymers in different battery technologies as well as emerging technologies such as biofuel cells or thermoelectric cells. Finally, the emerging applications of redox polymers in medicine, in new technologies such as tissue engineering, drug delivery, actuators or bioelectronics will be presented.