Groundwater fills and flows in the pore-space, fractures, and conduits of geological formations beneath the Earth's surface, called aquifers. Groundwater is the Earth's largest non-frozen freshwater reservoir, accounting for more than 97% of the liquid freshwater stock. Groundwater is the world's most extracted natural resource with withdrawal rates in the range of 1000 km3/year, and about 70% of the pumped groundwater is used for agriculture worldwide. Groundwater is a reliable freshwater resource that moves slowly in the aquifer, providing vital benefits for billions of people worldwide. Groundwater supplies more than half of the drinking water; helps to grow food by supplying approximately 40% of irrigation water; accounts for about one third of freshwater supply for industrial activities; and supports groundwater dependent ecosystems in aquifers, soil, rivers, lakes, wetlands, coastal zones, and marine environments, providing numerous ecosystem services. In addition, groundwater is a geothermal energy resource, which can be used for heating and cooling in urban heat islands as an example. Moreover, groundwater can generally serve as a manageable buffer to droughts, surface water seasonal variations, and floods. However, there are growing concerns over unsustainable groundwater pumping exceeding natural and induced recharge along with groundwater contamination and salinization, and degradation of groundwater dependent ecosystems. For example, more than half of the largest aquifers on Earth are being depleted, given estimates derived from the GRACE satellite mission (Richey et al., 2015). In addition, sustainable management of groundwater resources is critical for climate adaptation strategies, as climate change and variability drive the aquifer recharge, and can change groundwater use. As the world population is nearing 8 billion, these essential benefits and growing concerns call for an action to ensure groundwater sustainability (Gleeson et al., 2019). This article shows that understanding the coupled water-ecology-human system in a participatory water governance framework is critical for sustainable groundwater management. In addition, the article discusses the sustainability challenges of coastal and karst aquifers as examples.