Trackers are used by the Gamemakers in the Hunger Games to keep a record on a tributes' location, state and if they're alive or not. They are inserted into the inner right or left arm of all tributes during their transport from the Capitol to the arena by a Capitol attendant. A syringe gun is injected into the arm and when activated the small metal tracker is placed deep under the skin. It is assumed that the presence of this tracker causes no immediate pain or discomfort to the tributes, but Katniss Everdeen once states that in her anxiety she 'pressed on her tracker even though it hurt'.
74th Hunger GamesIn The Hunger Games movie, before the tributes entered the arena of the 74th Hunger Games the trackers are inserted manually by a female Capitol attendant, while the tributes sit in the hovercraft's cargo bays. In the book, a current paralyzes the tributes prior to sitting down and the tracker is inserted.
75th Hunger Games
In Catching Fire, during the final day of the 75th Hunger Games as Katniss Everdeen and Johanna Mason were running to the beach with the wire, Johanna cut Katniss' tracker out of her arm. At the time Katniss thought Johanna was trying to kill her, but it is later revealed that she removed her tracker so the Capitol would be unable to track her after the Games when she was rescued by the rebels and brought to District 13.