The Pont du Gard is an ancient Roman aqueduct that crosses the Gardon River near the town of Vers-Pont-du-Gard in southern France. The Pont du Gard, built as three tiers of archways to bring water to the city of Nîmes, is the highest of all elevated Roman aqueducts, and one of the best preserved. It was added to UNESCO's list of World Heritage Sites in 1985 because of its historical importance.

The aqueduct bridge is part of the Nîmes aqueduct, a 31 mile system built in the first century AD to carry water from a spring at Uzes to the Roman colony of Nemausus (Nîmes). Because of the uneven terrain between the two points, the mostly underground aqueduct followed a long, winding route that called for a bridge across the gorge of the Gardon River. The bridge has three tiers of arches, stands 160 ft high, and descends a mere 1 in - a gradient of only 1 in 18,241 - while the whole aqueduct descends in height by only 41 ft over its entire length, which is indicative of the great precision that Roman engineers were able to achieve using simple technology. The aqueduct formerly carried an estimated 8,800,000 imp gallon of water a day to the fountains, baths and homes of the citizens of Nîmes. It may have been in use as late as the 6th century, with some parts used for significantly longer, but a lack of maintenance after the 4th century led to clogging by mineral deposits and debris that eventually choked off the flow of water.

After the Roman Empire collapsed and the aqueduct fell into disuse, the Pont du Gard remained largely intact, due to the importance of its secondary function as a toll bridge. For centuries the local lords and bishops were responsible for its upkeep, in exchange for the right to levy tolls on travelers using it to cross the river, although some of its stones were looted and serious damage was inflicted on it in the 17th century. It attracted increasing attention starting in the 18th century, and became an important tourist destination. It underwent a series of renovations between the 18th and 21st centuries, commissioned by the local authorities and the French state, which culminated in 2000 with the opening of a new visitor center and the removal of traffic and buildings from the bridge and the area immediately around it. Today it is one of France's most popular tourist attractions, and has attracted the attention of a succession of literary and artistic visitors.

The location of Nemausus (Nîmes) was somewhat inconvenient when it came to providing a water supply. Plains lie to the city's south and east, where any sources of water would be at too low an altitude to be able to flow to the city, while the hills to the west made a water supply route too difficult from an engineering point of view. The only real alternative was to look to the north and in particular to the area around Ucetia (Uzes), where there are natural springs.

The Nîmes aqueduct was built to channel water from the springs of the Fontaine d'Eure near Uzes to the castellum divisorum (repartition basin) in Nemausus. From there, it was distributed to fountains, baths and private homes around the city. The straight-line distance between the two is only about 12 mi but the aqueduct takes a winding route measuring around 31 mi. This was necessary to circumvent the southernmost foothills of the Massif Central, known as the Garrigues de Nîmes. They are difficult to cross, as they are covered in dense vegetation and garrigue and indented by deep valleys. It was impractical for the Romans to attempt to tunnel through the hills, as it would have required a tunnel of between 5 and 6 mi, depending on the starting point. A roughly V-shaped course around the eastern end of the Garrigues de Nîmes was therefore the only practical way of transporting the water from the spring to the city.

The Fontaine d'Eure, at 249 ft above sea level, is only 56 ft higher than the repartition basin in Nîmes, but this provided a sufficient gradient to sustain a steady flow of water to the 50,000 inhabitants of the Roman city.  It is estimated that the aqueduct supplied the city with around 40,000 cubic metres (8,800,000 imp gal) of water a day that took nearly 27 hours to flow from the source to the city

The water arrived in the castellum divisorum at Nîmes - an open, shallow, circular basin 5.5 m in diameter by 1 m deep. It would have been surrounded by a balustrade within some sort of enclosure, probably under some kind of small but elaborate pavilion. When it was excavated, traces of a tiled roof, Corinthian columns and a fresco decorated with fish and dolphins were discovered in a fragmentary condition. The aqueduct water entered through an opening 3 ft 11 in wide, and ten large holes in the facing wall, each 16 in wide, directed the water into the city's main water pipes. Three large drains were also located in the floor, possibly to enable the nearby amphitheater to be flooded rapidly to enable mock naval battles to be held.

The spring still exists and is now the site of a small modern pumping station. Its water is pure but high in dissolved calcium carbonate leached out of the surrounding limestone. This presented the Romans with significant problems in maintaining the aqueduct, as the carbonates precipitated out of the water during its journey through the conduit. This caused the flow of the aqueduct to become progressively reduced by deposits of calcareous sinter. Another threat was posed by vegetation penetrating the stone lid of the channel. As well as obstructing the flow of the water, dangling roots introduced algae and bacteria that decomposed in a process called biolithogenesis, producing concretions within the conduit. It required constant maintenance by circitores, workers responsible for the aqueduct's upkeep, who crawled along the conduit scrubbing the walls clean and removing any vegetation.

Much of the Nîmes aqueduct was built underground, as was typical of Roman aqueducts. It was constructed by digging a trench in which a stone channel was built and enclosed by an arched roof of stone slabs, which was then covered with earth. Some sections of the channel are tunneled through solid rock. In all, 22 mi of the aqueduct was constructed below the ground. The remainder had to be carried on the surface through conduits set on a wall or on arched bridges. Some substantial remains of the above-ground works can still be seen today, such as the so-called "Pont Rue" that stretches for hundreds of meters around Vers and still stands up to 25 ft high. Other surviving parts include the Pont de Bornegre, three arches carrying the aqueduct 56 ft across a stream; the Pont de Sartanette, near the Pont du Gard, which covers 105 ft across a small valley; and three sections of aqueduct tunnel near Sernhac, measuring up to 217 ft long. However, the Pont du Gard is by far the best preserved section of the entire aqueduct.

Built on three levels, the Pont Bridge is 161 ft high above the river at low water and 899 ft long. Its width varies from 30 ft at the bottom to 9.8 ft at the top. The three levels of arches are recessed, with the main piers in line one above another. The span of the arches varies slightly, as each was constructed independently to provide flexibility to protect against subsidence. Each level has a differing number of arches.

The construction of the aqueduct has long been credited to the Roman emperor Augustus' son-in-law and aide, Marcus Vipsanius Agrippa, around the year 19 BC. At the time, he was serving as aedile, the senior magistrate responsible for managing the water supply of Rome and its colonies. Espérandieu, writing in 1926, linked the construction of the aqueduct with Agrippa's visit to Narbonensis in that year. Newer excavations suggest the construction may have taken place between 40 and 60 AD. Tunnels dating from the time of Augustus had to be bypassed by the builders of the Nîmes aqueduct, and coins discovered in the outflow in Nîmes are no older than the reign of the emperor Claudius (41-54 AD). On this basis, a team led by Guilhem Fabre has argued that the aqueduct must have been completed around the middle of the 1st century AD. It is believed to have taken about fifteen years to build, employing between 800 and 1,000 workers.

From the 4th century onwards, the aqueduct's maintenance was neglected as successive waves of invaders disrupted the region. It became clogged with debris, encrustations and plant roots, greatly reducing the flow of the water. The resulting deposits in the conduit, consisting of layers of dirt and organic material, are up to 20 in thick on each wall. An analysis of the deposits originally suggested that it had continued to supply water to Nîmes until as late as the 9th century but more recent investigations suggest that it had gone out of use by about the sixth century, though parts of it may have continued to be used for significantly longer.

Although some of its stones were plundered for use elsewhere, the Pont du Gard remained largely intact. Its survival was due to its use as a toll bridge across the valley. In the 13th century the French king granted the seigneurs of Uzes the right to levy tolls on those using the bridge. The right later passed to the Bishops of Uzes. In return, they were responsible for maintaining the bridge in good repair. However, it suffered serious damage during the 1620s when Henri, Duke of Rohan made use of the bridge to transport his artillery during the wars between the French royalists and the Huguenots, whom he led. To make space for his artillery to cross the bridge, the duke had one side of the second row of arches cut away to a depth of about one-third of their original thickness. This left a gap on the lowest deck wide enough to accommodate carts and cannons, but severely weakened the bridge in the process.

The Pont du Gard has been a tourist attraction for centuries. The outstanding quality of the bridge's masonry led to it becoming an obligatory stop for French journeymen masons on their traditional tour around the country, many of whom have left their names on the stonework. From the 18th century onward, particularly after the construction of the new road bridge, it became a famous staging-post for travelers on the Grand Tour and became increasingly renowned as an object of historical importance and French national pride.

The bridge has had a long association with French monarchs seeking to associate themselves with a symbol of Roman imperial power. King Charles IX of France visited in 1564 during his Grand Tour of France and was greeted with a grand entertainment laid on by the Duc d'Uzes. Twelve young girls dressed as nymphs came out of a cave by the riverside near the aqueduct and presented the king with pastry and preserved fruits. A century later, Louis XIV and his court visited the Pont du Gard during a visit to Nîmes in January 1660 shortly after the signature of the Treaty of the Pyrenees. In 1786 his great-great-great-grandson Louis XVI commissioned the artist Hubert Robert to produce a set of paintings of Roman ruins of southern France to hang in the king's new dining room at the Palace of Fontainebleau, including a picture depicting the Pont du Gard in an idealized landscape. The commission was meant to reassert the ties between the French monarchy and the imperial past. Napoleon III, in the mid-19th century, consciously identified with Augustus and accorded great respect to Roman antiquities; his patronage of the bridge's restoration in the 1850s was essential to its survival.

By the 1990s the Pont du Gard had become a hugely popular tourist attraction but was congested with traffic – vehicles were still allowed to drive over the 1743 road bridge – and was cluttered with illegally built structures and tourist shops lining the river banks. As the architect Jean-Paul Viguier put it, the "appetite for gain" had transformed the Pont du Gard into "a fairground attraction". In 1996 the General Council of the Gard département began a major four-year project to improve the area, sponsored by the French government, in conjunction with local sources, UNESCO and the EU. The entire area around the bridge was pedestrianised and a new visitor centre was built on the north bank to a design by Jean-Paul Viguier. The redevelopment has ensured that the area around the Pont du Gard is now much quieter due to the removal of vehicle traffic, and the new museum provides a much improved historical context for visitors. The Pont du Gard is today one of France's top five tourist attractions, with 1.4 million visitors reported in 2001.