Astromech droids, also referred to as astro droids or mechs, were a type of repair droid that served as an automated mechanic on starships. These compact droids used tool-tipped appendages stored in recessed compartments. Many starfighters relied on astromech copilots to control flight and power distribution systems. Sitting in an astromech socket exposed to space, the droid, in addition to its piloting duties, could calculate hyperspace jumps and perform simple repairs. Most astromechs were only able to communicate in writing, conveyed via another computer system, or through a special code of clicks, bleeps, and similar sounds, known as binary.

However, certain other droids, such as C-3PO, could understand the binary code of astromechs, and often translated from binary to other languages.

Astromech droids averaged 0.93 meters in height, while R-series droids from the R2 line forward had a height of just over a meter. However, the astromech P2 stood around the same height as a human.

R-series droids had three wheeled legs. The later BB-series astromech droids were spherical, giving them the ability to roll. Some astromechs, such as OC-JK14, were moved on treadwells. Though this style was once popular in the Outer Rim Territories, it had fallen out of favor by 35 ABY.