The introduction of deep brain stimulation (DBS) as a treatment for 
medication-refractory essential tremor in the late 1980s revealed, for the first 
time, that "chronically" implanted brain hardware had the potential to modulate 
neurologic function with surprisingly low morbidity. Over time, the therapeutic 
promise of DBS has become evident in Parkinson's disease and dystonia. In some 
experienced centers, complex tremor disorders, such as posttraumatic Holmes 
tremor and the tremor of multiple sclerosis, are being increasingly targeted. 
More recently, other indications, including obsessive-compulsive disorder, 
Tourette's syndrome, major depression, and chronic pain, have been proposed. As 
the field has expanded, our knowledge about potential cognitive side effects of 
DBS has also expanded. This article reviews the current knowledge regarding the 
impact of stimulation of the subthalamic nucleus, globus pallidus internus, and 
ventralis intermedius nucleus of the thalamus on symptoms in essential tremor, 
Parkinson's disease, and dystonia. Also discussed are the emerging targets, what 
is known about the cognitive sequelae of DBS, and what has been learned about 
the complications and therapeutic failures.