Roller chain or bush roller chain is the type of chain drive most commonly used for transmission of mechanical power on many kinds of domestic, industrial and agricultural machinery, including conveyors, wire- and tube-drawing machines, printing presses, cars, motorcycles, and bicycles. It consists of a series of short cylindrical rollers held together by side links. It is driven by a toothed wheel called a sprocket. It is a simple, reliable, and efficient means of power transmission.

Sketches by Leonardo da Vinci in the 16th century show a chain with a roller bearing. In 1800, James Fussell patented a roller chain on development of his balance lock and in 1880 Hans Renold patented a bush roller chain.

There are two types of links alternating in the bush roller chain. The first type is inner links, having two inner plates held together by two sleeves or bushings upon which rotate two rollers. Inner links alternate with the second type, the outer links, consisting of two outer plates held together by pins passing through the bushings of the inner links. The "bushingless" roller chain is similar in operation though not in construction; instead of separate bushings or sleeves holding the inner plates together, the plate has a tube stamped into it protruding from the hole which serves the same purpose. This has the advantage of removing one step in assembly of the chain.

The roller chain design reduces friction compared to simpler designs, resulting in higher efficiency and less wear. The original power transmission chain varieties lacked rollers and bushings, with both the inner and outer plates held by pins which directly contacted the sprocket teeth; however this configuration exhibited extremely rapid wear of both the sprocket teeth and the plates where they pivoted on the pins. This problem was partially solved by the development of bushed chains, with the pins holding the outer plates passing through bushings or sleeves connecting the inner plates. This distributed the wear over a greater area; however the teeth of the sprockets still wore more rapidly than is desirable, from the sliding friction against the bushings. The addition of rollers surrounding the bushing sleeves of the chain and provided rolling contact with the teeth of the sprockets resulting in excellent resistance to wear of both sprockets and chain as well. There is even very low friction, as long as the chain is sufficiently lubricated. Continuous, clean, lubrication of roller chains is of primary importance for efficient operation, as is correct tensioning.

Many driving chains (for example, in factory equipment, or driving a camshaft inside an internal combustion engine) operate in clean environments, and thus the wearing surfaces (that is, the pins and bushings) are safe from precipitation and airborne grit, many even in a sealed environment such as an oil bath. Some roller chains are designed to have o-rings built into the space between the outside link plate and the inside roller link plates. Chain manufacturers began to include this feature in 1971 after the application was invented by Joseph Montano while working for Whitney Chain of Hartford, Connecticut. O-rings were included as a way to improve lubrication to the links of power transmission chains, a service that is vitally important to extending their working life. These rubber fixtures form a barrier that holds factory applied lubricating grease inside the pin and bushing wear areas. Further, the rubber o-rings prevent dirt and other contaminants from entering inside the chain linkages, where such particles would otherwise cause significant wear.

There are also many chains that have to operate in dirty conditions, and for size or operational reasons cannot be sealed. Examples include chains on farm equipment, bicycles, and chain saws. These chains will necessarily have relatively high rates of wear.

Many oil-based lubricants attract dirt and other particles, eventually forming an abrasive paste that will compound wear on chains. This problem can be reduced by use of a "dry" PTFE spray, which forms a solid film after application and repels both particles and moisture.

Chains operating at high speeds comparable to those on motorcycles should be used in conjunction with an oil bath. For modern motorcycles this is not possible, and most motorcycle chains run unprotected. Thus, motorcycle chains tend to wear very quickly relative to other applications. They are subject to extreme forces and are exposed to rain, dirt, sand and road salt.