Stonehenge, the most famous prehistoric MEGALITH (standing-stone monument) in Europe, lies 13 km (8 mi) north of Salisbury, England. Excavations and radiocarbon dating have revealed that Stonehenge had an exceptionally long history of use as a ceremonial or religious center or both. In period I (c.2800 BC), the site was enclosed by a circular ditch with an internal bank and an entrance on the northeast side. Inside the bank on the inner side of the ditch was a ring of 56 pits--named Aubrey holes for their 17th-century discoverer, John Aubrey--that later were used for the burial of cremated bodies. Outside the entrance, the builders erected the huge, upright Heelstone and a timber gate. In period II (c.2100 BC), people of the BEAKER CULTURE built an earthwork approach road, now called the Avenue, to the entrance of the bank and ditch. They also set up within the earlier ring a double circle of bluestone MENHIRS (large, rough-hewn standing stones), which came originally from the Preseli Mountains of southwestern Wales. Both the Avenue and the double circle were orientated toward the summer solstice sunrise.

In period III (from c.2000 BC), builders erected in the center of the site a circle of 30 sarsen-stone uprights 30.5 m (100 ft) in diameter and capped by a continuous ring of sarsen lintels. This circle, in turn, encloses a horseshoe-shaped setting of five sarsen trilithons--formations in which two uprights support a lintel. All of the sarsen stones, which were transported about 30 km (20 mi) from the Marlborough Downs, were dressed to shape with stone hammers and jointed together. The precision with which this complex was laid out and the architectural refinements it displays are unparalleled in the other megalithic structures of northwestern Europe. 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 (c.1550 BC) in the circle and horseshoe whose remains survive today. In period IV (c.1100 BC), the Avenue was extended to the River Avon, 2 km (1.25 mi) from Stonehenge.

Among the megalithic monuments of Europe, Stonehenge is unique because of its long period of use and the precision of its plan and its architectural details. The long-held thesis that Stonehenge was a Druid temple is untenable, because the Druids did not appear in Britain until a few hundred years before the Christian era. In recent years many attempts have been made to interpret Stonehenge as a prehistoric astronomical observatory, but the site is now so ruined, and so much restored, that any attempt to ascertain its original alignments must rely principally on guesswork. All that can be said with confidence is that from period II onward the structure's axis of symmetry pointed roughly in the direction of the sunrise at the summer solstice.