Diplomats and spies are bound together by ‘dangerous liaisons’ in late medieval France and Burgundian France, as the power and administration of their prince is growing. Nobility with broad social networks, international merchants, and educated clerks travelling around the world can be used as spies or ambassadors. The situation triggers off great mistrust made obvious by their rejection from permanent embassies. Princes and ambassadors have to learn the art of fighting spies to achieve their diplomatic goals and, to that end, must use their very methods: fake mails, ‘burn after reading’ orders, and secret diplomacy become the usual business. A good prince as well as a good diplomat must be a spymaster: he needs to intoxicate, corrupt, and recruit informants. While doing so, he takes the risk of discrediting diplomacy and the prince himself, liable of his schemes. Every prince must act on the borderline between secret and open communication in a very complex game where his honour and power are at stake.