The janggu (or janggo), sometimes called seyogo (slim waist drum), is the most representative drum in traditional Korean music, and consists of an hourglass-shaped body with two heads made from animal skin. The two heads produce sounds of different pitch and timbre, which when played together are believed to represent the harmony of man and woman.

The first depiction of the instrument is on a bell belonging to the Silla (57 BC–935 AD) period and in a mural painting of the same period in Goguryeo (37 BC–935 AD) tomb. The oldest Korean historical records about an hourglass-shaped drum may be traced to the reign of King Munjong (1047–1084) of Goryeo as a field instrument. The Goryeo-sa (1451), or History of Goryeo, records 20 janggu as a gift from the Song Dynasty Emperor Huizong to the Goryeo Court in Gaeseong in 1114.

The janggu may have evolved from the yogo ("waist drum"), another similar but smaller Korean drum still in use today. The yogo may have originated from the idakka, an Indian instrument introduced into Korea from India through China during the Silla (57 BC–935 AD) period. Mural paintings in the tomb of Jipanhyun of Goguryeo, pictures at the Gameun Temple, and bronze relics of Buddha, made in the second year of King Mun (682) all depict the yogo . It was during the time of Goryeo that the Janggu grew to its present-day size.

The drum was made from a hollow, hourglass-shaped wooden body of either porcelain, tile, metal, gourd, tinned sheet, or wood. Paulownia wood was most popular because it is the lightest and best resonating material, producing beautiful sounds.

Jorongmok is the round tube in the middle connecting the left and right side of the hourglass-shaped body. The size of the jorongmok determines the quality of the tone: the wider the tube, the deeper and huskier it sounds; the narrower the tube, the harder and snappier it sounds.

The two skin heads are lapped onto metal hoops placed over the open ends of the body and secured by rope counter-loops. The left head (book side) is covered with a thick cowhide, horsehide, or deerskin to produce deep and low tones. The right side (chae side) is covered with either dog skin or a lighter horsehide to produces higher tones.

There are two kinds of beating sticks (chae). The gungchae is shaped like a mallet with a round head. The handle is made from bamboo root, boiled and straightened out and the head is made from hardwood such as birch or antler. Modern gungchae may also be made from plastic; this variety is normally used by beginning musicians. The yeolchae is always made from bamboo.

Traditionally the janggu is played using yeolchae on the right hand high pitch area with the bare hand on the low pitch area. Pungmul players for a number of folk songs and shamanistic rituals use this technique, though it is more common now to see the gungchae and yeolchae used together. 'Gungchae' is used to play the low pitch side. Janggu can be played on the floor as in traditional sanjo music or carried with a strap on the shoulder.