This chapter examines intracranial electroencephalography (iEEG), a rare but powerful technique offering unparalleled insights into human brain function by recording electrical activity directly from the brain’s surface. It traces iEEG’s development from pioneering work by Penfield and Jasper in the 1950s to modern applications with up to 1,024 recording channels. The chapter outlines the two primary surgical approaches, stereo EEG with depth electrodes and electrocorticography with surface grids, and explains how these techniques achieve both high temporal (millisecond) and spatial (millimeter) resolution by bypassing the signal-dampening effects of skull and scalp. Particular attention is given to high-gamma-power signals (70–200 Hz), which reflect neuronal firing with exceptional signal-to-noise ratios. The chapter addresses methodological considerations including electrode localization, signal processing, and data interpretation challenges unique to recording from epilepsy patients. It balances discussion of iEEG’s remarkable advantages, such as direct access to neuronal activity across cortical layers and network nodes along with its limitations, including restricted accessibility, sparse sampling, and the clinical constraints that dictate electrode placement. The ethical framework governing this invasive research methodology is emphasized throughout.