A central challenge for grid operators is matching electricity supply to demand, especially when the electricity supply is composed in part of intermittent resources. Several system options could help balance electricity supply and demand given different mixes of intermittent, baseload and load-following generation capacity; of these, electricity storage is especially interesting. If electricity storage could be deployed widely, grids of any size could sustain a wide range of profiles of intermittent and baseload power. Currently, most installed electricity storage worldwide is pumped hydro. Flywheels, compressed air and batteries represent interesting technologies that could provide grid-scale storage, especially if technology costs come down. A significant amount of storage R&D worldwide is appropriately focused on lowering these costs, but more is needed. Ultimately, storage will only achieve high levels of penetration if it can compete for service provision in electricity markets, and policy adjustments are needed in many countries to ensure this is the case. Published by Elsevier Ltd.