Groundwater wells run dry when infiltration or recharge cannot replenish groundwater as quickly as it is being extracted. During prolonged droughts and periods of high evaporation, groundwater depletion and reduced well yields can lead to groundwater drought (i.e. when wells run dry). This condition is often worsened by excessive groundwater pumpingGroundwater pumping, which further lowers the water table. As opposed to meteorological, agricultural, or even hydrological drought, which are primarily driven by climate change, groundwater drought results from both climate change and unsustainable groundwater withdrawals, making it more complex to measure, monitor, and manage. While the concept of groundwater drought is not entirely new, it remains significantly underexplored in both research and policy compared to other forms of drought. Its occurrence is only now gaining attention as intensified groundwater pumping and declining recharge are causing aquifers to dry up in several regions. Consequently, the processes driving groundwater drought and its associated impacts are still poorly understood, underscoring a critical and emerging area of inquiry within hydroclimatic science and groundwater management. This chapter outlines the principles of groundwater drought and highlights the roles of climate variability and human extraction in driving it. Across many regions, overuse of groundwater is depleting aquifers and lowering water tables, threatening ecological functions and water security. Droughts often lead to increased reliance on groundwater, putting additional pressure on aquifers, especially when surface water shrinks or declines. The chapter reviews key hydrological processes, especially recharge and discharge dynamics, and examines how groundwater drought affects these processes. It distinguishes groundwater drought from other forms or types (e.g. meteorological, agricultural, hydrological, and socio-economic droughts, etc.) of drought, emphasizes its widespread environmental and socioeconomic impacts, which include loss of groundwater-dependent ecosystems (wetlands, springs, rivers, and phreatophytic vegetation), and declines in unique aquatic biodiversity, as well as land subsidence. This chapter also stresses the need for integrated satellite monitoring and sustainable groundwater management to address this growing challenge. Such monitoring capabilities help close data gaps and improve the prediction of groundwater drought, particularly in high-need areas where in situ data are scarce or entirely absent.