After Cyrus' death, his son Cambyses ruled for eight years (530-522 BC) and continued his father's work of conquest, making significant gains in Egypt. A power struggle followed Cambyses' death and, despite his tenuous connection to the royal line, Darius was declared king (ruled 522-486 BC). He was to be arguably the greatest of the Persian rulers.

Darius' first capital was at Susa, and he started the building programme at Persepolis. He built a canal between the Nile and the Red Sea, a forerunner of the modern Suez Canal. He improved the extensive road system, and it is during his reign that mention is first made of the Royal Road (shown on map), a great highway stretching all the way from Susa to Sardis with posting stations at regular intervals.

Major reforms took place under Darius. Coinage was introduced - the daric (gold coin) and the shekel (silver coin) - and he greatly increased the efficiency of administration. The Old Persian language appears for the first time in royal inscriptions, written in a specially adapted version of cuneiform.