A pulley is a simple machine consisting of a wheel on an axle or shaft that may have a groove between two flanges around its circumference. A rope, cable, belt, or chain usually runs over the wheel and inside the groove, if present.

Pulley systems are used to change the direction of an applied force, transmit rotational motion, or realize a mechanical advantage in either a linear or rotational system of motion. In a "block and tackle" system, multiple pulleys allow a small force (effort) to lift a much larger weight (load).