Battery energy storage systems are the essential backbone of the renewable energy transition, storing solar & wind power for use when the sun is not shining & the wind is not blowing. Lithium-ion battery costs have fallen over ninety percent in a decade, with Tesla, BYD, LG Chem, Samsung, & Panasonic leading the market. Flow & sodium-sulfur batteries address long-duration storage needs. Regulatory gaps & high upfront costs remain the primary barriers to faster deployment of this critical clean energy technology.