Stonehenge is, according to The Oxford Encyclopedic English Dictionary, Clarendon Press, Oxford (1991), a 'unique megalithic monument on Salisbury Plain in Wiltshire, England'. It is situated 30 km south of the Avebury site and 13 km northwest of Salisbury. It was built in prehistoric times, beginning about 3100 BC (although the oldest signs of human activity, found under the car park, date from about 7000 BC). The monument consists of a monumental circular setting of large standing stones surrounded by an earthwork. Today, much of Stonehenge is ruined. Many of the stones have been pilfered.

The construction of Stonehenge can be divided into four periods. The original construction, made around 3100-2300 BC, was a circular ditch of about 100 metres in diameter with an internal bank, and a north-eastern entrance. Just inside the earth bank were 56 holes forming a ring. Probably also dating to this time are the four Station Stones (only two of which survive) and, on the north-east side, an earthwork which runs from the break in the bank and ditch. The first stones at the site that probably also date from this period are the Slaughter Stone (now fallen) and the Heel Stone. They were erected outside the entrance to the site.

Around 2000 BC the earthwork approach road to the entrance of the bank and ditch, now called the Avenue, was built by people of the Beaker Culture. 80 blocks of bluestone (spotted dolerite) were transported by the same people from a quarry almost 200 miles away in the Prescelly Mountains. It is surmized that these blocks were transported by way of rafts along the Welsh coast and up local rivers, finally to be dragged overland to the site. These stones or menhirs were erected forming two concentric circles. At some point this construction was dismantled and work began on the final phase of the site. The bluestones were moved within the circle and the gigantic stones that give Stonehenge its distinctive look were installed. Some of these stones weigh as much as 26 tons. Ten upright stones arranged as five freestanding pairs with a single lintel (the so-called trilithons) were placed in the shape of a horseshoe. The trilithons were then enclosed within a circle of about 33 metres in diameter comprised originally of 30 neatly trimmed upright sandstone blocks (known today as sarsens). These stones, which stand on average 4 metres above the ground, are about 2 metres wide, and 1 metre thick, supported a continuous ring of sarsen lintels (held in place by tongue-and-groove joints). The final element that was added was the altar block, a large block of green sandstone from South Wales that was placed in front of one of the trilithons. The 35-ton heel stone was possibly placed during the second period. Its placement was one of the most sophisticated accomplishments of that age and provides the best evidence that early people used astronomy. On Midsummer Day (June 24 then, now June 21) a person standing in the center of the circle can see the sun rise directly above the heel stone.

From 2000 - 1550 BC a circle of 30 sarsen-stone uprights (weighing up to 50 tons) 30.5 m in diameter and capped by a continuous ring of sarsen lintels was erected in the centre of the site. This circle surrounded a horseshoe-shaped setting of five sarsen trilithons. After transporting the sarsen stones from Marlborough Downs, 30 km away, the stones were shaped and jointed together with stone hammers. Other changes involved adding, moving, and rearranging stones that had been used during the second period. Some of the bluestones were later reerected in the center in an oval structure that contained at least two miniature trilithons, and holes were dug for the rest to be set in two concentric circles (the so-called Y and Z holes) outside the sarsen circle. This plan was abandoned unfinished, however, and the bluestones were finally rearranged (ca. 1550 BC) in the circle and horseshoe whose remains survive today. Finally the Avenue was extended to the River Avon around 1550-1100 BC.

Several mysteries surround the Stonehenge monument, like: how on earth did they build that, and even more interesting: what was it for?

Stonehenge's axis points roughly in the direction of the sunrise at the summer and winter solstices. Some scientists believe that the people who built and used Stonehenge were able to foretell eclipses of the sun and moon by their positions in relation to the monument. It can probably be safely assumed that this orientation is not accidental. Nobody knows however whether timekeeping and prediction of eclipses, equinoxes and such was the purpose of Stonehenge, or if it was built that way for symbolic reasons (like a mosque, that has the mihrab to point out the direction of Mecca, while the purpose of a mosque is not specifically to know where Mecca is).

There have been several stories and theories over the centuries. One of them is told by the twelfth century writer, Geoffrey of Monmouth, in his History of the Kings of Britain. His story is based on the legend of King Arthur and states that Merlin brought the stones to the Salisbury Plain from Ireland by magic. According to Geoffrey of Monmouth, the stones of the Giant's Ring stone circle were originally brought from Africa to Ireland by giants. The stones were located on "Mount Killaraus" and were used as a site for performing rituals and for healing.They were moved from Ireland to England as a memorial to 300 British noblemen who were slaughtered by a treacherous Saxon leader sometime in the fifth century.

Another belief that was widely spread in the 17th century was that Stonehenge was a Druid temple. As pointed out by Diomedes, they certainly did not build it, although they may well have used it. Early belief states that the Stonehenge monument was built as a temple for sky worship, but no proof has been found for this.