When we think of the Greek poleis, we immediately think of a world of temples, philosophers, artists and the city of Athens, the cradle of democracy; in reality, we can already see forms of espionage in Ancient Greece. As demonstrated by the English historian Christopher Andrew in his book The Secret World, Greece was among the first civilisations to recognise the strategic importance of intelligence in war and politics. One of the earliest mythological examples is that of the Trojan horse, made immortal by Homer and gone down in history as a masterpiece of psychological warfare and indirect espionage. Coming to historical evidence, however, one cannot fail to mention a crucial source: Thucydides. In his Peloponnesian War, the Athenian historian recounted how the city of Athens employed proxenoi, i.e. pro-Athenian foreign citizens, who worked as moles and informers in enemy cities. An example of a master of strategic deception was Themistocles, who succeeded in misleading the Persians: through his double agent Sicinnus, he had false information delivered to Xerxes in order to lure the Persian fleet into the straits of Salamis, where it was later defeated in the famous battle of 480 BC. Moreover - and this was to be a constant throughout the following centuries - merchants and travellers were the perfect personalities to act as spies. Thanks to their mobility, their trade and their ability to penetrate many contexts, they collected and transmitted crucial information: troop movements, political crises, secret alliances and much more. As the boundary between ambassadors and spies was very blurred in the Greek world, the information network was as crucial as it was invisible. Ultimately, ancient Greece was a veritable laboratory of early intelligence practices, where cunning and the ability to infiltrate and obtain key information made the difference between victory and defeat for a poleis.