Which part of the brain controls balance and coordination?

The cerebellum, whose name means little brain, tucks in beneath the back of the cerebrum and handles the fine timing of movement. It does not decide to reach for a cup; the motor cortex does that. The cerebellum takes the intended movement and compares it against streams of information from the inner ear, the eyes, muscles and joints, then corrects the action in flight so it lands smoothly rather than overshooting. This is why cerebellar damage causes a distinctive pattern of clumsy, jerky movement, unsteady walking and difficulty with rapid alternating actions, without any paralysis. Alcohol depresses cerebellar function, which is exactly why roadside sobriety tests ask people to walk a line or touch their nose. The cerebellum also stores learned motor patterns, so practised skills like cycling or playing an instrument become automatic. Despite occupying a small fraction of the brain's volume, it contains more neurons than the rest of the brain combined.

The inner ear's fluid-filled semicircular canals supply much of the balance information the cerebellum works with.