Classical Athens, known contemporaneously simply as Athens (Ancient Greek: Ἀθῆναι, Athênai [atʰɛ̂ːnai̯]; Modern Greek: Αθήναι, Athine [aˈθine]), was the major urban centre of the notable polis (city-state) of the same name during the classical period (480–323 BC) of ancient Greece, located in Attica, Greece, leading the Delian League in the Peloponnesian War against Sparta and the Peloponnesian League. Athenian democracy was established in 508 BC under Cleisthenes following the tyranny of Isagoras. This system remained remarkably stable, and with a few brief interruptions, it remained in place for 180 years, until 322 BC (amid the aftermath of the Lamian War). The peak of Athenian hegemony was achieved during the Age of Pericles in the 440s and 430s BC.

In the classical period, Athens was a centre for the arts, learning, and philosophy, and it was the home of Plato's Academy and Aristotle's Lyceum. Athens was also the birthplace of Socrates, Plato, Pericles, Aristophanes, Sophocles, and many other prominent philosophers, writers, and politicians of the ancient world. It is widely referred to as the cradle of Western civilization and as the birthplace of democracy, largely due to the impact of its cultural and political achievements during the 5th and 4th centuries BC on the rest of the then-known European continent.

Hippias, son of Peisistratus, had ruled Athens jointly with his brother, Hipparchus, from the death of Peisistratus in about 527. Following the assassination of Hipparchus in about 514, Hippias took on sole rule, and in response to the loss of his brother, became a worse leader who was increasingly disliked. Hippias exiled 700 of the Athenian noble families, amongst them Cleisthenes's family, the Alchmaeonids. Upon their exile, they went to Delphi, and Herodotus says they bribed the Pythia always to tell visiting Spartans that they should invade Attica and overthrow Hippias. That supposedly worked after a number of times, and Cleomenes I led a Spartan force to overthrow Hippias, which succeeded, and instated an oligarchy. Cleisthenes disliked the Spartan rule, along with many other Athenians, and so made his own bid for power. The result was democracy in Athens, but considering Cleisthenes's motivation for using the people to gain power, as without their support, he would have been defeated, and so Athenian democracy may be tainted by the fact its creation served greatly the man who created it. The reforms of Cleisthenes replaced the traditional four Ionic "tribes" (phyle) with ten new ones, named after legendary heroes of Greece and having no class basis, which acted as electorates. Each tribe was in turn divided into three trittyes (one from the coast; one from the city and one from the inland divisions), while each trittys had one or more demes, depending on their population, which became the basis of local government.

The tribes each selected fifty members by lot for the Boule, the council that governed Athens on a day-to-day basis. The public opinion of voters could be influenced by the political satires written by the comic poets and performed in the city theaters. The Assembly or Ecclesia was open to all full citizens and was both a legislature and a supreme court, except in murder cases and religious matters, which became the only remaining functions of the Areopagus. Most offices were filled by lot, although the ten strategoi (generals) were elected.

The silver mines of Laurion contributed significantly to the development of Athens in the 5th century BC, when the Athenians learned to prospect, treat, and refine the ore and used the proceeds to build a massive fleet, at the instigation of Themistocles.

In 499 BC, Athens sent troops to aid the Ionian Greeks of Asia Minor who were rebelling against the Persian Empire. This provoked two Persian invasions of Greece, both of which were repelled under the leadership of the soldier-statesmen Miltiades and Themistocles. In 490 the Athenians, led by Miltiades, prevented the first invasion of the Persians, guided by king Darius I, at the Battle of Marathon. In 480 the Persians returned under a new ruler, Xerxes I. The Hellenic League led by the Spartan King Leonidas led 7,000 men to hold the narrow passageway of Thermopylae against the 100,000–250,000 army of Xerxes, during which Leonidas and 300 other Spartan elites were killed. Simultaneously the Athenians led an indecisive naval battle off Artemisium. However, that delaying action was not enough to discourage the Persian advance, which soon marched through Boeotia, setting up Thebes as their base of operations, and entered southern Greece. That forced the Athenians to evacuate Athens, which was taken by the Persians, and seek the protection of their fleet. Subsequently, the Athenians and their allies, led by Themistocles, defeated the Persian navy at sea in the Battle of Salamis. Xerxes had built himself a throne on the coast in order to see the Greeks defeated. Instead, the Persians were routed. Sparta's hegemony was passing to Athens, and it was Athens that took the war to Asia Minor. The victories enabled it to bring most of the Aegean and many other parts of Greece together in the Delian League, an Athenian-dominated alliance.

Pericles – an Athenian general, politician and orator – distinguished himself above the other personalities of the era, men who excelled in politics, philosophy, architecture, sculpture, history and literature. He fostered arts and literature and gave to Athens a splendor which would never return throughout its history. He executed a large number of public works projects and improved the life of the citizens. Hence, this period is often referred to as "Age of Pericles." Silver mined in Laurium in southeastern Attica contributed greatly to the prosperity of this Athenian Golden Age.

During the time of the ascendancy of Ephialtes as leader of the democratic faction, Pericles was his deputy. When Ephialtes was assassinated by personal enemies, Pericles stepped in and was elected general, or strategos, in 445 BC; a post he held continuously until his death in 429 BC, always by election of the Athenian Assembly. The Parthenon, a lavishly decorated temple to the goddess Athena, was constructed under the administration of Pericles.