In the transition to sustainable energy, hydrogen emerges as a crucial carrier, addressing the intermittency of renewable sources. However, its storage poses challenges due to its unique properties, notably in surface-based systems. Underground hydrogen storage (UHS) presents a viable solution, drawing on natural gas storage methodologies while offering enhanced capacity, safety, and reduced environmental impact. This chapter provides an in-depth analysis of UHS, exploring its operational mechanisms, geological requirements, and the adaptation of existing infrastructures. It highlights the significance of geological formations such as caverns, aquifers, and depleted hydrocarbon reservoirs, evaluating their suitability based on various factors including rock and mineral properties as well as the relevant operational dynamics. Moreover, the chapter provides an overview that integrates scientific, engineering, and logistical perspectives of UHS, offering insights into project planning and the global context of UHS initiatives, thereby underscoring its potential and challenges as a key component in the energy transition.