King Darius I, also known as Darius the Great, was the third king of the Achaemenid Empire and ruled from 522 to 486 BCE. He is best known for his administrative reforms, which helped to unify and manage the vast territories of the empire. Darius introduced a standardized system of weights and measures, as well as a new currency, which facilitated trade and economic stability. Darius I is also famous for his construction projects, including the grand palace at Persepolis and the Royal Road, which improved communication and transportation across the empire. His reign marked a period of expansion, as he successfully conquered regions such as Egypt and parts of India.