The water table or phreatic surface is the surface where the water pressure is equal to atmospheric pressure. The water table is also often erroneously defined as the surface that separates the phreatic from the vadose zone, below which all rocks are saturated with water.

A large amount of water within a body of sand or rock below the water table is called an aquifer, and the ability of rocks to store such groundwater is dependent on their porosity and permeability.

The practice of drilling wells to extract groundwater is dependent on understanding the water table. Because wells must reach the water table, its depth determines the minimum depth of a viable well, and thus the feasibility of drilling it.

In Australia, because of deforestation, farming and urbanisation*, salt water tables have risen near to the surface or to ground level, endangering plant life, farm economy and urban development as the soil grows more salty. These salt water tables were formed millions of years ago when parts of Australia were covered by ocean. As the continent dried, the water became trapped underground.

In areas with a high water table, tunnels and basements are less common due to the hydrostatic pressure exerted by the water, and the risk of flooding.