Derailleurs (from the french derailler, literally to "derail") come in many shapes and sizes. Most modern bicycles have two: a front derailleur and a rear derailleur. Front and rear derailleurs are quite different in appearance, but serve the same function: to guide a moving bicycle chain from one chainring (or gear) to the next.

Front derailleurs are typically attached to a bicycle frame's seat tube, and hang above the front chainrings. The front derailleur is simply a narrow metal channel through which the chain passes as it moves from the rear sprockets to the front chainrings. When the gearshift lever is moved, the channel shifts to one side, forcing the chain onto a higher or lower chainring. Front derailleurs require a sensitive touch to shift properly; excessive movement can cause the chain to derail from the chainrings altogether.

Rear derailleurs are more complicated. They typically are attached to the rear dropouts of the bicycle frame, and hang below the rear sprocket. Most rear derailleurs contain one or more small toothed pulleys on which the chain rides as it passes through the derailleur. The guiding action of the pulleys is necessary to exert enough lateral force on the chain to shift across the entire range of the rear sprocket assembly, which may span from three to seven chainrings.