Most renewable energy sources depend on the sun and so vary with time and ambient conditions. Hence a consistent supply of renewable energy requires energy storage. The main approaches to storing renewable energy are described and quantified. Pumped hydro, compressed air and flywheels are discussed. Storing heat in the fabric of buildings and hot water using sensible heat are described. The increasing importance of phase change materials to store energy through latent heat is recognized. Battery technology is developing very fast; the principles of lithium-ion batteries are explained, together with their advantages and disadvantages. The various materials currently used for the positive electrode are listed. The electrochemistry of various battery technologies is summarized as well as how a large number of cells are connected to form are a useful store of energy. The principle of flow batteries is demonstrated and approaches to the estimation of the lifetime of a lithium-ion battery discussed. The chapter is supported by 10 examples, 16 questions with answers and full solutions in the accompanying online material. Further reading and online resources are identified.