The pathophysiologic abnormalities that underlie essential tremor (ET) are difficult to decipher because autopsy studies reveal no gross or microscopic abnormalities. Electrophysiologic studies are consistent with a central source of tremorogenic oscillation. The inferior olive and cerebellum are implicated by PET studies. Harmaline tremor in animals shares many features with ET, and the inferior olive has been identified as the source of oscillation in this animal model. Therefore, a disturbance of olivocerebellar rhythmicity is at present the most popular hypothesis for the etiology of ET. Although electrophysiologic tests are available that are helpful in the diagnosis of ET, a gold-standard test or biologic marker for ET is still lacking.