Hydraulics is a science and engineering subject dealing with the mechanical properties of liquids. Hydraulics is part of the more general discipline of fluid power. The word "hydraulics" originates from the Greek word ὑδραυλικός (hydraulikos) which in turn orginates from ὕδραυλος meaning water organ which in turn comes from ὕδρω (water) and αὐλός (pipe). The earliest masters of this art were Hero of Alexandria and Ctesibius. The ancient engineers focused on sacral and novelty uses of hydraulics, rather than practical applications. Ancient Sinhalese used hydraulics in many applications, in the Ancient kingdoms of Anuradhapura, Polonnaruwa etc. The discovery of the principle of the valve tower, or valve pit, for regulating the escape of water is credited to Sinhalese ingenuity more than 2,000 years ago. By the first century A.D, several large-scale irrigation works had been completed. Macro- and micro-hydraulics to provide for domestic horticultural and agricultural needs, surface drainage and erosion control, ornamental and recreational water courses and retaining structures and also cooling systems were in place in Sigiriya, Sri Lanka.

One of the founders of modern hydraulics was Benedetto Castelli, a student of Galileo Galilei.

Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on the engineering uses of fluid properties. Hydraulic topics range through most science and engineering disciplines, and cover concepts such as pipe flow, dam design, fluid control circuitry, pumps, turbines, hydropower, computational fluid dynamics, flow measurement, river channel behavior and erosion.