Accordion is a box-shaped musical instrument which belongs to the hand-held bellows-driven category. It is a portable instrument which is also sometimes referred to as a squeezebox. The oldest name for the instruments belonging to this group is the harmonika. Today the name accordion is more common. The accordion as we know it today was patented by Cyrill Demian, which concerned `automatically coupled chords on the bass side`. The accordion has been used most often in folk music in Europe and the Americas. It became very popular in Russia. An instrument called the bayan, with its own unique keyboard layout, was developed from it, for which various Russian composers have written music. Very often, the accordion has been referred to as a one man band because it needs no accompanying instrument. A person who plays the accordion is called an accordionist.

The Accordion is played as a bellow instrument. It is played by compressing or expanding a bellows whilst pressing buttons or keys, causing valves, called pallets, to open, which allow air to flow across strips of brass or steel, called reeds, which vibrate to produce sound inside the body. Accordions have piano keys and push buttons. The right hand plays these melody keys and buttons. The left hand plays the accompaniment consisting of bass and pre-set chord buttons. The Accordionist hangs his instrument from a belted sling and holds it to the body, which makes it easy to play standing up.

On the slightly large instruments all kinds of tone combinations can be obtained through register stop keys that introduce a whole bank of tonal effects, including a tremolo. The button accordion has three rows of descant buttons. The accordion models with five rows do not have a wider compass. The extra rows are duplicates which have been made in order to facilitate ease of playing. Depending on the instrument, the bass side has between 24 and 140 bass notes. Only the first rows have individual notes, the remainder being chord keys or buttons, such as for major thirds, minor thirds, and sevenths. On the larger instruments, special tone combinations are available.

Quite a few twentieth century composers have written music for the accordion, including Paul Dessau, Serge Prokofiev, Virgil Thomson, Roberto Gerhardt, and Frans Vuursteen. The Dutchman, Harry Mooten, is a virtuoso accordionist who plays Bach arrangements as well as light music. It is commonly associated with busking. Some popular music acts also make use of the instrument. Additionally, the accordion is sometimes used in both solo and orchestra performances of classical music.

In the Indian music scenario, the accordion is often used in the film industry as an accompaniment for Hindi film songs. It is also used for stage shows and pop music concerts. The most popular form of the accordion used here is the piano accordion.