Book excerpt: The global energy system is undergoing a profound transformation from a system based mainly on fossil fuels to a low-carbon one based on variable renewable energy (VRE), such as wind power and solar power, to achieve the 2050 Paris Agreement. By 2050, solar and wind power, with more than 14,500 GW installed capacity, would account for three-fifths of global electricity generation. This transformation comes with significant challenges since high VRE shares will greatly increase system flexibility requirements for balancing supply and demand. Accordingly, all sectors of the power system need to unlock further requisite flexibility through technology, business, and policy innovations, including power supply, transmission, distribution, storage, and demand.