EEG machines record the brain's electrical activity through scalp electrodes, detecting synchronized neuronal firing as voltage fluctuations (microvolts) amplified ~10,000x for display. Key components include 19-256 Ag/AgCl electrodes (10-20 system: Fp1/Fp2, F3/F4, etc.), differential amplifiers (reject 50/60Hz noise), analog filters (0.5-70Hz bandpass), A/D converters (≥256Hz sampling), and software for montages (bipolar/referential), FFT power spectra, and spike detection. Used for epilepsy diagnosis (3Hz spike-wave), sleep staging, coma prognosis, and intraoperative monitoring.