This chapter focuses on high frequency oscillations (HFOs), which are also known as transient oscillations in the broad 80 to 6000 Hz frequency range. HFOs typically represent a normal physiological activity and pathological phenomenon found in epileptic tissue. Technologies like intracranial EEG (iEEG) and scalp electrodes aid with recording HFOs and ripples that became an important biomarker of epilepsy and epileptogenesis. The chapter highlights the significance of identifying ripples and fast ripples of neuronal activity, which are neurophysiological events that can be defined by features such as association with sharp waves and particular laminar distribution. It then expounds on the differences between physiological HFOs and pathological HFOs.