A bell /ˈbɛl/ (^ⓘ) is a directly struck idiophone percussion instrument. Most bells have the shape of a hollow cup that when struck vibrates in a single strong strike tone, with its sides forming an efficient resonator. The strike may be made by an internal "clapper" or "uvula", an external hammer, or—in small bells—by a small loose sphere enclosed within the body of the bell (jingle bell).

Bells are usually cast from bell metal (a type of bronze) for its resonant properties, but can also be made from other hard materials. This depends on the function. Some small bells such as ornamental bells or cowbells can be made from cast or pressed metal, glass or ceramic, but large bells such as a church, clock and tower bells are normally cast from bell metal.

Bells intended to be heard over a wide area can range from a single bell hung in a turret or bell-gable, to a musical ensemble such as an English ring of bells, a carillon or a Russian zvon which are tuned to a common scale and installed in a bell tower. Many public or institutional buildings house bells, most commonly as clock bells to sound the hours and quarters.

Historically, bells have been associated with religious rites, and are still used to call communities together for religious services. Later, bells were made to commemorate important events or people and have been associated with the concepts of peace and freedom. The study of bells is called campanology.

Bell is a word common to the Low German dialects, cognate with Middle Low German belle and Dutch bel but not appearing among the other Germanic languages except the Icelandic bjalla which was a loanword from Old English. It is popularly but not certainly related to the former sense of to bell (Old English: bellan, 'to roar, to make a loud noise') which gave rise to bellow.

The earliest archaeological evidence of bells dates from the 3rd millennium BCE, and is traced to the Yangshao culture of Neolithic China. Clapper-bells made of pottery have been found in several archaeological sites. The pottery bells later developed into metal bells. In West Asia, the first bells appear in 1000 BCE. The earliest metal bells, with one found in the Taosi site and four in the Erlitou site, are dated to about 2000 BCE. With the emergence of other kinds of bells during the Shang dynasty (c. 1600 – c. 1050 BCE), they were relegated to subservient functions; at Shang and Zhou sites, they are also found as part of the horse-and-chariot gear and as collar-bells of dogs. By the 13th century BCE, bells weighing over 150 kilograms (330 pounds) were being cast in China. After 1000 CE, iron became the most commonly used metal for bells instead of bronze. The earliest dated iron bell was manufactured in 1079, found in Hubei Province.

Bells west of China did not reach the same size until the 2nd millennium CE. Assyrian bells dated to the 7th century BCE were around 4 inches high. Roman bells dated to the 1st and 2nd century CE were around 8 inches high. The book of Exodus in the Bible notes that small gold bells were worn as ornaments on the hem of the robe of the high priest in Jerusalem. Among the ancient Greeks, handbells were used in camps and garrisons and by patrols that went around to visit sentinels. Among the Romans, the hour of bathing was announced by a bell. They also used them in the home, as an ornament and emblem, and bells were placed around the necks of cattle and sheep so they could be found if they strayed. As late as the 10th century CE, European bells were no higher than 2 feet in height.

In the western world, the common form of bell is a church bell or town bell, which is hung within a tower or bell cote. Such bells are either fixed in a static position ("hung dead") or mounted on a beam (the "headstock") so they can swing to and fro. Bells that are hung dead are normally sounded by hitting the sound bow with a hammer or occasionally by pulling an internal clapper against the bell.