Deep underground energy storage (DUES) refers to the storage of energy or energy carriers such as oil, natural gas, hydrogen, compressed air, CO2, and helium in deep underground space. DUES, as a large-scale energy storage technology, has broad application prospects. It is a key technology to support the large-scale development of clean energy and ensure oil and gas supply security. It has advantages such as large storage capacity, cleanliness and environmental protection, safety and reliability, and good economy. It can make full use of underground space to achieve large-scale and efficient storage of various forms of energy, which is of great significance for optimizing energy structure, promoting clean energy production, and ensuring energy security. It is an important choice for implementing the strategy of sustainable green energy development. Deep rock salt formations are one of the preferred geological bodies for energy storage, with low permeability, low porosity, good rheology, and self-healing properties. Salt cavern UGS involves the intersection of multiple disciplines such as mining engineering, geotechnical engineering, mechanics, underground engineering, geological engineering, geophysics, and geochemistry, which is a hotspot and challenge in the international engineering and academic community. This chapter analyzed the state of DUES, especially in China and its challenges, as well as the main content of this book.