In this chapter, we start to discuss how an electrolyte interacts with an electrode and the emergence of an important component: the interface. The basic properties of an interface, an electrified double layer, are described. We start with two extreme examples of interfaces, ideally non-polarizable and ideally polarizable interfaces, and discuss why a mercury surface is a perfect platform to understand how the interface assembles and disassembles charges. A few typical interface models are introduced. We then continue to study how a charge could travel across an interface, i.e. electrodics, and derive the famous Butler–Volmer equation, followed by discussions of the success and limitations of that equation.