Immortals, or Persian Immortals, was the name given by the Greek historian Herodotus to a 10,000-strong unit of elite heavy infantry in the Achaemenid army. They served in a dual capacity, operating as an imperial guard and contributing to the ranks of the standing army. The force mainly consisted of Persians, along with Medes and Elamites. Essential questions regarding the unit's history and organization remain unanswered due to the lack of authoritative sources.