Hydrogen is being recognized as a potential energy carrier that could decarbonize several sectors, among them are few hard-to-abate ones such as transportation, heating, chemical industries, refineries, and power sector. The increase in renewable energy capacity worldwide to combat the ill effects of fossil fuels heightened the need for large-scale, long-duration energy storage  to address the intermittency of solar and wind power. While conventional storage are suitable for short-term and small-scale applications, underground hydrogen storage (UHS) provides a viable solution for seasonal and grid-scale storage in the form of hydrogen energy. This chapter discusses underground hydrogen storage concepts, geological storage options, and their technical, economic, and safety considerations. The suitability of various geological formations such as salt caverns, depleted oil and gas reserves, saline aquifers, and some non-conventional formations, for hydrogen storage is understood in terms of storage capacity, cushion gas requirements, sealing integrity, hydrogen losses, geochemical stability, and operational challenges. Additionally, the history and the development of these formations is technically discussed through key parameters such as site selection, reservoir properties, fluid-fluid, solid-fluid interactions present in the underground storage systems. Overall, the crucial role of underground hydrogen storage in facilitating the integration of large-scale renewable energy for reliable and low-carbon energy ecosystem is highlighted.