This final chapter considers the need for energy storage methods for the effective implementation of renewable energy sources. Our energy use is not constant in time but shows daily, weekly and seasonal fluctuations. This applies to our need for both electrical energy and thermal energy. While energy produced by fossil fuels can largely follow fluctuations in demand, this is not necessarily true for sources of renewable energy. The present chapter reviews the temporal fluctuations that are characteristic of the various sources of renewable energy and illustrates the need for methods to store energy in order to most efficiently accommodate demand. As well, the low energy density and non-portable nature of many types of renewable energy makes energy storage a requirement for their use in applications such as remote energy systems and transportation.