The article presents the functioning of spies' secret underworld during the reign of Louis XIV in France. First, it defines the concept of espionage. Next, it describes its different aspects: political, military and economic. The duty of spies was to obtain both general and classified information. Secret information was most appreciated by the Fereign Ministry. Intelligence service activities involved special rules, precise guidelines and unique tactics. Being a high-risk profession, spying was well-paid and attracted adventurous people who loved challenge. They mainly included middle nobility, but also clergy, foreigners and women. In order to avoid the risk of discovery, spies employed special methods to hide their messages, such as invisible ink, secret code or cipher, lemon juice cryptography, or double envelopes. They used various methods to secure their hidden intelligence network, themselves and their knowledge. Based on the experience built during the reign of Louis XIV, the secret service developed the foundations for nowadays practices.