Energy is important to all of us, for a variety of reasons, but primarily because it can be useful. It can be found in a number of different forms. One readily recognizes the concepts of potential energy and kinetic energy, as well as the chemical energy in fuels, thermal energy as heat, the kinetic energy in wind and moving water, and magnetic and electrical energy in a variety of guises. But energy is often present in one form, whereas we want to use it in another form. This requires some kind of conversion mechanism or transducer device. Furthermore, energy may be available in amounts and at times and places that are different from those when and where we want to utilize it. Thus, methods to store and transport energy from place to place can be of great importance.This text has to do with the storage of energy, and there are a number of different ways in which this can be done. The title indicates that it is about electrochemical energy storage. This may appear to be misleading, for there is actually no such thing as electrochemical energy. What it actually means is that we shall deal with the general topic of the use of electrochemical means to convert between two different types of energy: electrical energy and chemical energy. This involves the use of electrochemical devices that act as transducers, for they convert between electrical and chemical quantities – energies, potentials, and fluxes. Such electrochemical transduction systems are often called galvanic cells, or in more common parlance, batteries. Electrical energy can be stored in electric or magnetic fields; mechanical energy can be stored in devices such as flywheels, and thermal energy can be stored in the form of heat. But the magnitudes of these forms of energy are all relatively small and the methods for their conversion into other forms are relatively unwieldy. Much larger amounts of energy can be present in the form of chemical species. This can be relatively attractive, for it can be relatively inexpensive and efficient in terms of the amount of energy stored per unit volume or weight. Thus, storage in chemical form is often a useful intermediate stage, holding energy for later use in other forms, such as electrical, heat, light, or mechanical energy.