Electroencephalography (EEG) examines the functional state of the brain. High-frequency oscillations (HFOs) in the ripple (80-200/250 Hz) and fast ripple (200/250-500/600 Hz) bands have recently been attracting attention, and their recording has been enabled by advancements in digital EEG techniques. The detection of HFOs was previously limited to intracranial EEG, but fast oscillations (FOs) in the gamma (40-80 Hz) and ripple bands can now be detected over the scalp. HFOs and FOs have been shown to be related to epileptogenicity in intracranial EEG and scalp EEG, respectively. A large number of FOs are found in the scalp EEGs of pediatric patients with various epileptic encephalopathies, particularly West syndrome. FOs are suggested to be a biomarker of the epileptogenic cortical region in epilepsy surgery. FOs are detectable even in patients with idiopathic focal epilepsies, including benign epilepsy with centrotemporal spikes and Panayiotopoulos syndrome, who are not generally candidates for operation. The detection of HFOs and FOs may provide clues to the pathophysiology of epilepsy and the relationship between HFOs and cognitive dysfunction.