Accumulators are storage vessels that hold fluid under pressure as energy to be released on demand, and are commonly used in fluid power systems to improve performance. When used in hydraulic circuits and machinery, they are sometimes referred to as hydraulic accumulators or hydropneumatic accumulators. Accumulators are also used to eliminate pulsations for a smooth fluid flow and protect systems from hydraulic shock or water hammer. The most common design has an internal bladder that separates stored process fluid from a compressed gas charge.

Accumulators are primarily used for pulsation dampening in process fluid lines, which is crucial for accurate metering and protection of sensitive process equipment. While the terms "hydraulic" and "hydropneumatic" accumulators are often interchanged, hydropnuematic accumulators are more often utilized in metering pump systems, chemical injection systems, process sampling systems, and clean-in-place (CIP) systems whereas hydraulic accumulators can be found in hydraulic actuator systems for process valves, emergency shutdown systems, and backup power systems for critical equipment.