The uneven bars or asymmetric bars is an artistic gymnastics apparatus. It is made of a steel frame. The bars are made of fiberglass with wood coating, or less commonly wood. The English abbreviation for the event in gymnastics scoring is UB or AB, and the apparatus and event are often referred to simply as "bars". The bars are placed at different heights and widths, allowing the gymnast to transition from bar to bar. A gymnast usually adds white chalk to the hands so that they can grip the bar better.

Uneven bars used in international gymnastics competitions must conform to the guidelines and specifications set forth by the International Gymnastics Federation Apparatus Norms brochure. Several companies manufacture and sell bars, including AAI in the United States, Jannsen and Fritsen in Europe, and Acromat in Australia.

Many gyms also have a single bar or a set of uneven bars over a loose foam pit or soft mat to provide an additional level of safety when learning new skills.

Measurements depend on the setting of the bar. Measurements of the bars are provided by the Fédération Internationale de Gymnastique (FIG) in the Apparatus Norms brochure.

Routines usually involve a lot of strength, jumping, and swings. They are generally composed of a set of skills, each worth a particular value.

The uneven bars apparatus originally consisted of men's parallel bars set to different heights. Consequently, the bars were very close together, and gymnasts could transition from one to the other with little difficulty. Routines of the early 1950s chiefly consisted of simple circles, kips, and static balance elements and holds. In the late 1950s, the trend shifted toward fluid motion, and gymnasts began to perform routines composed of more difficult circles, kips, beats (bouncing the body off the low bar while hanging from the high bar), wraps (wrapping the body around the low bar while hanging from the high bar) and transitions. Release moves also began to come into play, although they were almost entirely limited to transitions between the low and high bars. In the late 1960s/early 1970s, companies began manufacturing uneven bars as a separate specific apparatus. The design was changed slightly to allow the bars to be adjustable, with tension cables that held the apparatus to the floor. As a result of this change, coaches could set the bars farther apart. Additionally, the bars' circumference decreased, allowing gymnasts to grasp and swing from them with greater ease.

As other events in gymnastics increased in difficulty, so did the uneven bars. Gymnasts and coaches began experimenting with elements, attempting more challenging dismounts, and adapting moves from men's horizontal bar. In 1972, Olga Korbut pioneered the Korbut Flip, the first high bar salto release move. Nadia Comăneci continued the trend with her original Comaneci salto at the 1976 Olympic Games in Montreal and advanced handstand elements four years later. The giant swing, the staple of high bar in men's artistic gymnastics (MAG), was also adopted into the women's Code of Points, and quickly became a basic uneven bars skill.

By the mid-1980s, routines had become so based on swing and release moves that the bars were moved even farther apart. The distance between bars increased even more as gymnasts developed complex transition elements that required space, such as the Pak salto.