This article introduces the principles and applications of biopotential electrode sensors in electrocardiography (ECG), electromyography (EMG), and electroencephalography (EEG) systems. Biopotential electrodes can monitor heart, muscle, and brain activity by measuring electric potentials on the surface of living tissue. These electrodes sense ion distribution on the tissue surface and convert ion current to electron current. An electrolyte solution/gel is applied to one side of the electrode in contact with the tissue, and a chemical reaction occurs at the interface between the electrolyte and the electrode. Ions are discharged into the electrolyte, while electrons carry charge through the lead wires. This technology finds wide applications in medical monitoring and diagnostics.