An injection well is used to place fluid underground into porous geologic formations. These underground formations may range from deep sandstone or limestone, to a shallow soil layer. Injected fluids may include water, wastewater, brine (salt water), or water mixed with chemicals. Injection effectively disposed of unwanted brine and preserved surface waters. In some formations injection enhanced the recovery of oil. Injection proved to be a safe and inexpensive option for the disposal of unwanted and often hazardous industrial byproducts.

Injection wells are used to inject water (by gravity or pressure) into an aquifer. They are commonly used in managed aquifer recharge schemes or groundwater remediation. It is used for managed aquifer recharge (both storage and recovery), and aquifer storage transfer and recovery, with stormwater, recycled water, and groundwater. 

The key issues in designing injection wells are: ensuring the injected water reaches only target aquifer; protecting the aquifer and aquitards from being damaged from over-pressurisation; minimising problems such as clogging and excessive recovery of aquifer material.