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 expanded the empire significantly, conquering regions such as Egypt and parts of India. Darius I is also famous for his construction projects, including the grand city of Persepolis, which served as the ceremonial capital. He introduced a standardized system of weights and measures and established a postal system, enhancing communication across the empire.