The castanets are a percussion instrument (idiophone), much used in Moorish music, Gypsy music, Spanish music and Latin American music. The instrument consists of a pair of concave shells joined on one edge by string. These are held in the hand and used to produce clicks for rhythmic accents or a ripping or rattling sound consisting of a rapid series of clicks. They are traditionally made of hardwood, although fibreglass is becoming increasingly popular.

In practice a player usually uses two pairs of castanets. One pair is held in each hand, with the string hooked over the thumb and the castanets resting on the palm with the fingers bent over to support the other side. Each pair will make a sound of a slightly different pitch. The higher pair, known as hembra (female), is usually held in the right hand, with the larger macho (male) pair held in the left.

Castanets are often played by singers or dancers, and are prominently used in flamenco music. The name (Spanish: castañuelas) is derived from the diminutive form of castaña, the Spanish word for chestnut, which they resemble. In Andalusia they are usually referred to as palillos (little sticks) instead, and this is the name by which they are known in flamenco.

The origins of the instrument are not known. The practice of clicking hand-held sticks together to accompany dancing is ancient, and was practiced by both the Greeks and the Egyptians. In more modern times, the bones and spoons used in Minstrel show and jug band music can also be considered forms of the castanet.

When used in an orchestral setting, castanets are sometimes attached to a handle, or mounted to a base to form a pair of machine castanets. This makes them easier to play, but also alters the sound, particularly for the machine castanets. It is possible to produce a roll on a pair of castanets in any of the three ways in which they are held. When held in the hand, they are bounced against the fingers and palm of the hand; on sticks, bouncing between fingers and the player's thigh is one accepted method. For a machine castanet, a less satisfactory roll is obtained by rapid alternation of the two castanets with the fingers.

Castanets are often used in classical music to evoke a Spanish atmosphere, as in Georges Bizet's opera, Carmen, or Emmanuel Chabrier's orchestral work España. They are also found in the "Dance of the Seven Veils" from Richard Strauss' opera Salome and in Richard Wagner's Tannhauser. An unusual variation on the standard castanets can be found in Darius Milhaud's Les Choëphores, which calls for castanets made of metal.