An aqueduct is a watercourse constructed to carry water from a source to a distribution point far away). In modern engineering, the term aqueduct is used for any system of pipes, ditches, canals, tunnels, and other structures used for this purpose). Aqueducts were used in ancient Greece, ancient Near East, and ancient Rome). The simplest aqueducts are small ditches cut into the earth, while much larger channels may be used in modern aqueducts). Aqueducts sometimes run for some or all of their path through tunnels constructed underground). Modern aqueducts may also make extensive use of pipelines, which are useful for transporting water over long distances when it needs to move over hills, or where open channels are poor choices due to considerations of evaporation, freezing, pollution, or environmental impact).

Historically, agricultural societies have constructed aqueducts to irrigate crops). Archimedes invented the water screw to raise water for use in irrigation of croplands). Aqueducts have also been important for the development of areas with limited direct access to fresh water sources. In ancient Rome, aqueducts helped keep drinking water free of human waste and other contamination and thus greatly improved public health in cities with primitive sewerage systems. Roman aqueducts were built throughout the empire, and their arches may still be seen in Greece, Italy, France, Spain, North Africa, and Asia Minor.

Modern aqueducts, although lacking the arched grandeur of those built by the Romans, greatly surpass the earlier ones in length and in the amount of water they can carry. Aqueduct systems hundreds of miles long have been built to supply growing urban areas. For example, New York Citys water supply comes from three modern aqueducts.