Electrodes are the interface between an energy source (electrical stimulator) and tissue (place where the stimulus has to be delivered). Electrode technology is a vast and difficult domain; it involves aspects of material science, biocompatibility, design techniques for geometry and implantation, fixation at tissue layers, electro-chemical reactions, physiology, cardiology, and all other body functions prone to be stimulated. All these aspects are briefly presented in this article. Biocompatibility and materials will be discussed related to electrodes, as this topic is also covered in other articles of this encyclopedia. Basically, electrodes are used externally (applied to the skin) as well as internally. Electrical stimulation is used for many functions. The major applications are in cardiology, for pain relief, for treatment of Parkinson's disease, for treatment of incontinence, and for gastroparesis. Cardiac applications will be highlighted more in this article.

Physiological, geometrical, and physical constraints must all be considered by bioengineers and medical doctors, thus leading to optimal sensing, stimulation, adequate thresholds, and long-term behavior after chronic clinical implantation.