The Seleucid Empire was a Hellenistic (or Ancient Greek) successor state of Alexander the Great's empire. At its greatest extent, the Empire covered central Anatolia, the Levant, Mesopotamia, Persia, Turkmenistan, Pamir and the Indus Valley.
Primarily, it was the successor to the Achaemenid Empire of Persia, and was followed there by the Islamic Caliphate (Rashidun Empire) conquest and rule, from 650s to 660s AD. Later on, much of this area became part of the Umayyad Empire and then the Abbasid Empire.
There were over 30 kings of the Seleucid dynasty from 323 to 63 BC.