Electrochemical reactions are those where the energy shortfall to drive a chemical reaction is supplied in the form of electrical energy. Alternatively, the energy released in the course of the reaction appears in the form of electrical energy. They constitute a very important class of reactions with immense industrial significance. They find applications in chlor alkali industries, metallurgical and metal-processing industries, wastewater treatment, specialty chemicals, energy storage, etc., to name a few. Their importance has led to the development of electrochemical engineering as an independent engineering discipline. Here, the reader is given a brief overview of the fundamentals of electrochemical reactions and the reactors for such applications.