The major motor disturbances in Parkinson's disease are thought to be caused by overactivity of the GABAergic internal segment of the globus pallidus (GPi), which acts as a "brake" on the motor thalamus and the cortical motor system to produce the slowness, rigidity, and poverty of movement characteristic of parkinsonian states. The goal of pallidotomy is to reduce this excessive inhibition on the motor system in patients with Parkinson's disease who continue to be significantly disabled despite pharmacotherapy. GPi can be identified with a high degree of precision through microelectrode recording and stimulation. Micro- and macrostimulation can be used to map the position of the optic tract and the internal capsule, two structures that are at risk with pallidotomy. Although the optimal lesion size and location within the pallidum is yet to be determined, unilateral lesions in the sensorimotor portion of GPi are associated with striking improvements in drug-induced involuntary movements, bradykinesia, tremor, and rigidity and to a lesser extent on gait and postural disturbances.