The iodine (I)-fueled plasma thruster is a form of space propulsion. The basic operating principle is momentum exchange. The working fluid is a plasma, which is typically created through electron bombardment of iodine vapor. The plasma is electrically accelerated to velocities greater than 10 km/s by an applied potential difference. In an iodine fueled Hall Effect Thruster (HET), the plasma is produced and accelerated in an open annular channel. In an iodine-fueled ion engine, the plasma is produced inside a cylinder capped by electrically biased acceleration grids. In either case, the ion beam is neutralized by a hollow cathode electron source. The iodine vapor is produced through sublimation at a reservoir located upstream of the thruster. This entry describes the basic principles of operation with a focus on HETs.