Period in Greek history dating from the death of Alexander the Great (323 BC) to Rome's conquest of Egypt (30 BC), when Greek culture spread as a result of Alexander's conquests. Kingdoms rather than city-states were the centres of power, but it was a time of vigorous intellectual activity in the sciences, mathematics and astronomy (above all in Alexandria) and in philosophy (Epicurus and the Stoics in Athens). The Hellenistic kingdoms were unable to offer a coordinated response to the expansion of Roman power, and they became part of the Roman empire.