Hydro, geothermal, tidal, and ocean-current electricity production can be steady for long periods; thus, these generators provide baseload (constant-output) electricity. However, wind, solar PV, and wave electricity outputs vary during the day and by season. As such, these electricity sources provide variable output. Given that solar and wind may end up supplying 90 percent or more of all WWS energy generation worldwide, on average, it is important to have electricity-storage technologies available to provide backup when solar and wind are unavailable. Storage also allows excess daytime WWS generation, for example, to be shifted to nighttime. Major electricity-storage options include existing hydroelectric dams, pumped hydroelectric storage, batteries, concentrated solar power coupled with thermal energy storage, flywheels, compressed-air energy storage, gravitational storage with solid masses, and green-hydrogen storage. This chapter discusses these technologies.