In music, a bow (/boʊ/) is a tensioned stick which has hair (usually horse-tail hair) coated in rosin (to facilitate friction) affixed to it. It is moved across some part (generally some type of strings) of a musical instrument to cause vibration, which the instrument emits as sound. The vast majority of bows are used with string instruments, such as the violin, viola, cello, and bass, although some bows are used with musical saws and other bowed idiophones.

A bow consists of a specially shaped stick with other material forming a ribbon stretched between its ends, which is used to stroke the string and create sound. Different musical cultures have adopted various designs for the bow. For instance, in some bows a single cord is stretched between the ends of the stick. In the Western tradition of bow making—bows for the instruments of the violin and viol families—a hank of horsehair is normally employed.

The manufacture of bows is considered a demanding craft, and well-made bows command high prices. Part of the bow maker's skill is the ability to choose high quality material for the stick. Historically, Western bows have been made of pernambuco wood from Brazil. However, pernambuco is now an endangered species whose export is regulated by international treaty, so makers are currently adopting other materials: woods such as Ipê (Tabebuia) and synthetic materials, such as carbon fiber epoxy composite and fiberglass.

For the frog, which holds and adjusts the near end of the horsehair, ebony is most often used, but other materials, often decorative, were used as well, such as ivory and tortoiseshell. Materials such as mother of pearl or abalone shell are often used on the slide that covers the mortise, as well as in round decorative "eyes" inlaid on the side surfaces. Sometimes "Parisian eyes" are used, with the circle of shell surrounded by a metal ring. The metal parts of the frog, or mountings, may be used by the maker to mark various grades of bow, ordinary bows being mounted with nickel silver, better bows with silver, and the finest being gold-mounted. (Not all makers adhere uniformly to this practice.) Near the frog is the grip, which is made of a wire, silk, or "whalebone" wrap and a thumb cushion made of leather or snakeskin. The tip plate of the bow may be made of bone, ivory, mammoth ivory, or metal, such as silver.

A bow maker or archetier typically uses between 150 and 200 hairs from the tail of a horse for a violin bow. Bows for other members of the violin family typically have a wider ribbon, using more hairs. There is a widely held belief among string players, neither proven nor disproven scientifically, that white hair produces a "smoother" sound and black hair (used mainly for double bass bows) is coarser and thus produces a "rougher" sound. Lower quality (inexpensive) bows often use nylon or synthetic hair, and some use bleached horse hair to give the appearance of higher quality. Rosin, or colophony, a hard, sticky substance made from resin (sometimes mixed with wax), is regularly applied to the bow hair to increase friction.

In making a wooden bow, the greater part of the woodworking is done on a straight stick. According to James McKean, "the bow maker graduates the stick in precise gradations so that it is evenly flexible throughout". These gradations were originally calculated by François Tourte, discussed below. To shape the curve or "camber" of the bow stick, the maker carefully heats the stick in an alcohol flame, a few inches at a time, bending the heated stick gradually—using a metal or wooden template to get the model's exact curve and shape.

The art of making wooden bows has changed little since the 19th century. Most modern composite sticks roughly resemble the Tourte design. Various inventors have explored new ways of bow-making. The Incredibow, for example, has a straight stick cambered only by the fixed tension of the synthetic hair.

Slightly different bows, varying in weight and length, are used for the violin, viola, cello, and double bass.