The lithium/iodine-polyvinylpyridine (PVP) battery has been in clinical use as a power source for pacemakers for 35 years. Since 1972, literally millions of lithium/iodine cells have seen clinical use, providing health-giving and life-saving therapy to patients throughout the world. Indeed, the battery has compiled an excellent record of reliability and safety. The cell reaction is the simple combination of lithium and iodine to form lithium iodide, but the chemistry of the cathode material is interesting and complicated.