This paper covers the measurement of biopotentials for diagnosis: the electrical voltages that can be measured from electrodes placed on the skin or within the body. Biopotentials include: the electrocardiogram (ECG), electroencephalogram (EEG), electrocortogram (ECoG), electromyogram (EMG), electroneurogram (ENG), electrogastrogram (EGG), action potential (AP), electroretinogram (ERG), electro-oculogram (EOG). This paper also covers skin conductance, pulse oximeters, urology, wearable systems and important therapeutic devices such as: the artificial cardiac pacemaker, defibrillator, cochlear implant, hemodialysis, lithotripsy, ventilator, anesthesia machine, heart-lung machine, infant incubator, infusion pumps, electrosurgery, tissue ablation, and medical imaging. It concludes by covering electrical safety. It provides future subjects for research such as a blood glucose sensor and a permanently implanted intracranial pressure sensor.