Essential tremor is a chronic neurological condition that presents as involuntary trembling in a part of the body. It occurs most often in the hands and arms, but may also affect the head, voice and legs. The tremor is often associated with purposeful movement, such as writing, buttoning clothes or holding a glass, or when the patient is maintaining a fixed position against gravity, such as when the arms are stretched out straight. The tremor is usually absent or minimal with rest and disappears during sleep.