Short-duration fluctuations of electricity supply and demand can cause instability in electricity grids and utility systems. These fluctuations are usually corrected using fast response time electrical energy storage devices that have high power and low storage capacity, hence they are never noticed by the end user. This chapter covers basic aspects of the most important electrical energy storage devices in this category, including supercapacitors and supercapatteries, superconducting magnetic energy storage devices, flywheels and synchronous condensers. Their fast response times and high efficiencies, particularly when storing energy for short durations, makes these devices very suitable for the continuous stabilisation of energy systems. Furthermore, the coupling of these devices to dc and ac electrical networks can provide natural instantaneous responses that stabilise such systems.