Sidney George Reilly (; c.1873 5 November 1925)known as "Ace of Spies"was a Russian-born adventurer and secret agent employed by Scotland Yard's Special Branch and later by the Foreign Section of the British Secret Service Bureau, the precursor to the modern British Secret Intelligence Service (MI6/SIS). He is alleged to have spied for at least four different great powers, and documentary evidence indicates that he was involved in espionage activities in 1890s London among Russian migr circles, in Manchuria on the eve of the Russo-Japanese War (190405), and in an abortive 1918 coup d'tat against Vladimir Lenin's Bolshevik government in Moscow.Reilly disappeared in Soviet Russia in the mid-1920s, lured by Cheka's Operation Trust. British diplomat and journalist R. H. Bruce Lockhart publicised his and Reilly's 1918 exploits to overthrow the Bolshevik regime in Lockhart's 1932 book Memoirs of a British Agent. This became an international best-seller and garnered global fame for Reilly. The memoirs retold the efforts by Reilly, Lockhart, and other conspirators to sabotage the Bolshevik revolution while still in its infancy.

The world press made Reilly into a household name within five years of his execution by Soviet agents in 1925, lauding him as a peerless spy and recounting his many espionage adventures. Newspapers dubbed him "the greatest spy in history" and "the Scarlet Pimpernel of Red Russia". The London Evening Standard described his exploits in an illustrated serial in May 1931 headlined "Master Spy". Ian Fleming used him as a model for James Bond in his novels (set in the early Cold War). Reilly is considered to be "the dominating figure in the mythology of modern British espionage".

Espionage, spying or intelligence gathering is the act of obtaining secret or confidential information (intelligence) from non-disclosed sources or divulging of the same without the permission of the holder of the information for a tangible benefit. A person who commits espionage is called an espionage agent or spy. Any individual or spy ring (a cooperating group of spies), in the service of a government, company, criminal organization, or independent operation, can commit espionage. The practice is clandestine, as it is by definition unwelcome. In some circumstances, it may be a legal tool of law enforcement and in others, it may be illegal and punishable by law.

Espionage is often part of an institutional effort by a government or commercial concern. However, the term tends to be associated with state spying on potential or actual enemies for military purposes. Spying involving corporations is known as industrial espionage.

One of the most effective ways to gather data and information about a targeted organization is by infiltrating its ranks. This is the job of the spy (espionage agent). Spies can then return information such as the size and strength of enemy forces. They can also find dissidents within the organization and influence them to provide further information or to defect. In times of crisis, spies steal technology and sabotage the enemy in various ways. Counterintelligence is the practice of thwarting enemy espionage and intelligence-gathering. Almost all nations have strict laws concerning espionage and the penalty for being caught is often severe. However, the benefits gained through espionage are often so great that most governments and many large corporations make use of it.