Electrochemical energy storage systems have the potential to make a major contribution to the implementation of sustainable energy. This chapter describes the basic principles of electrochemical energy storage and discusses three important types of system: rechargeable batteries, fuel cells and flow batteries. A rechargeable battery consists of one or more electrochemical cells in series. Electrical energy from an external electrical source is stored in the battery during charging and can then be used to supply energy to an external load during discharging. Two rechargeable battery systems are discussed in some detail: the lead–acid system, which has been in use for over 150 years, and the much more recent lithium system; sodium–sulfur and nickel–metal hydride systems are also briefly discussed. A fuel cell is an electrochemical cell in which the reactants supplying the energy are not stored in the cell itself but rather are continuously supplied to the electrodes from an external source. A common example is a hydrogen–oxygen fuel cell: in that case, the hydrogen and oxygen can be generated by electrolysing water and so the combination of the fuel cell and electrolyser is effectively a storage system for electrochemical energy. Both high- and low-temperature fuel cells are described and several examples are discussed in each case. A flow battery is similar to a conventional rechargeable battery in that it can be repeatedly charged and discharged. However, the energy storage material is dissolved in the electrolyte as a liquid and so can be stored in external tanks. Various types of flow batteries are available or under development. Three of the more important examples are discussed in some detail: the all-vanadium flow battery, the zinc–bromine hybrid flow battery and the all-iron slurry flow battery. Some other examples are also briefly mentioned. The choice of electrochemical storage system is highly dependent on the specific requirements of the project that is being considered, the associated upfront capital and lifetime expenditure costs and end-of-life, environmental and safety considerations.