In 478 BCE, Athens formed the Delian League to liberate Greek cities from Persian rule. But within a generation, this alliance of autonomous city-states had transformed into the Athenian Empire. This episode traces the League's evolution from its founding on the island of Delos, through the transfer of the treasury to Athens in 454 BCE, to the brutal suppression of revolts at Naxos and Thasos. We explore how the League's own mechanisms—tribute, military obligations, and the Athenian fleet—became tools of imperial control. Key figures like Aristides the Just, who set the initial tribute assessments, and Cimon, the popular general, are discussed alongside the structural changes that made empire-building possible. The episode also examines the so-called 'Tribute Lists' inscribed on stone, revealing the economic grip Athens held over its allies. By the eve of the Peloponnesian War, the Delian League had become an empire in all but name—a transformation that Thucydides famously called the real cause of the war with Sparta.