A booster pump is a machine which increases the pressure of a fluid. It may be used with liquids or gases, and the construction details vary depending on the fluid. A gas booster is similar to a gas compressor, but generally a simpler mechanism which often has only a single stage of compression, and is used to increase pressure of a gas already above ambient pressure. Two-stage boosters are also made. Boosters may be used for increasing gas pressure, transferring high pressure gas, charging gas cylinders and scavenging.

On new construction and retrofit projects, water pressure booster pumps are used to provide adequate water pressure to upper floors of high rise buildings. The need for a water pressure booster pump can also arise after the installation of a backflow prevention device (BFP), which is currently mandated in many municipalities^[where?] to protect the public water supplies from contaminants within a building entering the public water supply. The use of BFPs began after The Clean Water Act was passed. These devices can cause a loss of 12 PSI, and can cause flushometers on upper floors not to work properly. After pipes have been in service for an extended period, scale can build up on the inside surfaces which will cause a pressure drop when the water flows.

Booster pumps for household water pressure are usually simple electrically driven centrifugal pumps with a non-return valve. They may be constant speed pumps which switch on when pressure drops below the low pressure set-point and switch off when pressure reaches the high set-point, or variable speed pumps which are controlled to maintain a constant output pressure.

Constant speed pumps are switched on by a normally closed low-pressure switch and will content to run until the pressure rises to open the high pressure switch. They will cycle whenever enough water is used to cause a pressure drop below the low set point. An accumulator in the upstream pipeline will reduce cycling.

Variable speed pumps use pressure feedback to electronically control motor speed to maintain a reasonably constant discharge pressure. Most applications run off AC mains current and use an inverter to control motor speed.

Installations that provide water to highrise buildings may need boosters at several levels to provide acceptably consistent pressure on all floors. In such a case independent boosters may be installed at various levels, each boosting the pressure provided by the next lower level. It is also possible to boost once to the maximum pressure required, and then to use a pressure reducer at each level. This method would be used if there is a holding tank on the roof with gravity feed to the supply system.

Multi-story buildings equipped with fire sprinkler systems may require a large booster pump to deliver sufficient water pressure and volume to upper floors in the event of a fire. Such pumps are often powered by a diesel engine dedicated to this purpose. The engine needs a fuel tank and an automatic controller that will start the booster pump when it is needed. A small auxiliary electrically powered booster pump (called a "jockey pump") is often included in the system to maintain the sprinkler pipes at sufficient pressure, without requiring startup of the large diesel engine.

Any emergency system must be periodically tested and maintained to ensure its reliability. A diesel engine must be started and operated for testing, and a battery bank for the starting motor must be maintained or replaced periodically. In recent years, a larger electrical pump with substantial battery backup may be substituted for the diesel engine, reducing but not eliminating the need for maintenance.