Infantry in the Persian army, and more precisely in the army of the Persian state under the Achaemenid dynasty (from approx. 550 BC to approx. 330 BC), played an important role, performing various functions - from scouting to fighting in the open field in progress battles. Most often it was placed in the center of the Persian army's combat formation. Its exact number is only an estimate - the numbers given by the author of "The Acts" of Herodotus are rather perceived as unrealistic. It is assumed, however, that the number of Persian infantry during Xerxes' expedition to Greece (480-479 BC) could be from 100,000 to 200,000. people. However, these are still estimates. Undoubtedly, the elite of the Persian infantry was the Immortals unit, always composed of 10,000, recruited from the native Persians and Medes. This unit is certainly certified in sources during the reign of Cambyses (533-520 BC), and perhaps it was formed by his father - Cyrus the Great (559-530 BC). The soldiers of this formation were very well trained, had high morale and were armed with spears, bows and probably short swords. On the other hand, they had marginal protective equipment, which brought a deplorable effect during the battles at Marathon (490 BC) or Thermopylae (480 BC). It was not until the Greco-Persian wars that the soldiers of this formation probably received lamellar armor.