A freewheel or overrunning clutch is a device in a transmission that disengages the driveshaft from the driven shaft when the driven shaft rotates faster than the driveshaft. An overdrive is sometimes mistakenly called a freewheel, but is otherwise unrelated.

The condition of a driven shaft spinning faster than its driveshaft exists in most chain-driven bicycles when the rider stops pedaling. In a specialized fixed-gear bicycle (that lacks a freewheel) the rear wheel drives the pedals around.

An opposite condition exists in an automobile with a manual transmission going downhill, or any situation where the driver takes their foot off the gas pedal (closing the throttle) but the clutch is left out (and the transmission remains engaged). Instead of the engine driving the wheels (through the transmission), the wheels will drive the engine, possibly at a higher RPM. Pure freewheeling in an automobile is pushing the clutch in and releasing the throttle, disengaging the connection between the engine and transmission and allowing the engine to idle while the wheels turn at whatever pace gravity and momentum propel them.

In a two-stroke engine, this can be catastrophic—as many two stroke engines depend on a fuel/oil mixture for lubrication, a shortage of fuel to the engine starves oil from the cylinders, and the pistons can soon seize, causing extensive damage. Saab used a freewheel system in their two-stroke models for this reason and maintained it in the Saab 96 V4 and early Saab 99 for better fuel efficiency.

The simplest freewheel device consists of two saw-toothed, spring-loaded discs pressing against each other axially with the toothed sides together, like a ratchet but with the usual stationary part also rotating. Rotating in one direction, the saw teeth of the drive disc lock with the teeth of the driven disc, making it rotate at the same speed. If the drive disc slows down or stops rotating, the teeth of the driven disc slip over the drive disc teeth and continue rotating, producing a characteristic clicking sound proportionate to the speed difference of the driven gear relative to that of the (slower) driving gear.

A more sophisticated and rugged design has spring-loaded steel rollers inside a driven cylinder. Rotating in one direction, the rollers lock with the cylinder making it rotate in unison. Rotating slower, or in the other direction, the steel rollers just slip inside the cylinder.

Bicycles use freewheels to allow the cyclist to coast without pedaling. Rotating either the wheel or cassette in the direction that produces the clicking sound causes the pawl to easily slide up and over the gently sloped edges of the teeth. This process is sometimes informally referred to as "slipping." In this scenario, the cassette rotates independently of the rear wheel. When the cyclist stops pedaling, the ratchet slips as the wheel continues to rotate while the cassette stops, producing the clicking noise. Consequently, a bicycle will not move in reverse if the cyclist pedals backwards. When the cassette or wheel is rotated in the opposite direction, the pawl catches against the steeper-sloped edges of the teeth, creating a lock. As the cyclist pedals forward, the cassette spins forward causing the pawl to catch against the steep slope of the teeth and drive the rear wheel in the forward direction.

Most bicycle freewheels use an internally step-toothed drum with two or more spring-loaded, hardened steel pawls to transmit the load. More pawls help spread the wear and give greater reliability although, unless the device is made to tolerances not normally found in bicycle components, simultaneous engagement of more than two pawls is rarely achieved.