The Hellenistic period (323–31 BC), bracketed by Alexander's death and the Battle of Actium, witnessed the creation of a Greek-speaking cosmopolitan culture that spread from Egypt to Bactria and fundamentally shaped the subsequent development of Christianity, Judaism, Islam, and Western science.

The successor kingdoms — Ptolemaic Egypt, the Seleucid Empire, Antigonid Macedonia, and smaller kingdoms like Pergamon and Pontus — combined Greek governing elites with indigenous populations in ways that varied considerably. Egypt under the Ptolemies maintained Egyptian religious institutions and adapted Greek rule to Egyptian expectations; the Seleucid Empire governed a more diverse territory with greater administrative variety.

Hellenistic science represents the most concentrated period of scientific advance in the ancient world. Eratosthenes calculated the circumference of the earth to within a few percent using shadow measurements at different latitudes. Aristarchus proposed a heliocentric model of the solar system that was not accepted — the geocentric model better fit available observations and philosophical assumptions — but was technically correct. Archimedes' work in mathematics and engineering remains the most impressive individual scientific achievement of antiquity.

The Library of Alexandria, under the patronage of the Ptolemaic kings, assembled the largest collection of texts in the ancient world and supported the scholarly tradition of textual criticism and commentary that preserved much of classical Greek literature. The Mouseion (Museum), attached to the Library, was effectively the first state-funded research institution in history. The question of when and how the Library was destroyed has been much debated; the honest answer is that it declined gradually over centuries, not in a single catastrophic fire.