Following US accusations that Russian spies tipped the vote in President-elect Donald Trump's favor — by stealing Democratic Party information — here are some of the most brazen operations in the West.
The subject of last year's thriller "Bridge of Spies" starring Mark Rylance, Rudolf Abel was the fake identity of a Soviet intelligence officer captured by FBI agents in New York in 1957.
He was exchanged for shot-down American pilot Francis Gary Powers in 1962 on the Glienicke Bridge which linked West Berlin with Soviet-controlled Potsdam.
Abel, whose real name was William Fisher, was sent by the KGB to the United States in 1948 and lived in New York, posing as an artist and photographer while helping to coordinate a network of spies smuggling nuclear secrets to the Soviet Union.
He communicated with his superiors through "dead drops" in New York — pre-arranged locations used to pass information — and was informed of the arrival of an assistant from Moscow with a thumbtack left on a signpost in the city's Central Park.
Julius and Ethel Rosenberg, who were executed for conspiracy to commit espionage in 1953 following a controversial trial, were part of that network.
Julius Rosenberg was accused of smuggling USdefensee secrets with the help of his wife while working on American military technology during World War II.
The judge who convicted them said the information they passed on helped the Soviet Union develop a nuclear bomb at the beginning of the Cold War.
A former senior British intelligence officer, Kim Philby was revealed in 1963 to be Britain's biggest Cold War traitor as a member of the spy ring now known as the Cambridge Five.
Philby and the others — all upper-class men embedded in the British establishment — were recruited to spy for the Soviet Union during their time at Cambridge University in the 1930s and were undetected for years.
Following his exposure in 1963, he fled to the Soviet Union where he died in 1988 at the age of 76.
Previously unseen footage of Philby giving a lecture to spies in then communist East Germany in 1981 was broadcast by the BBC earlier this year.
In the grainy video, Philby revealed how he befriended MI6 archivists so as to take home secret files that would then be copied by his Soviet contact.
"That I did regularly, year in, year out," he said.
Soviet revolutionary Leon Trotsky was murdered in 1940 in Mexico City with an ice pick to the head in an operation by the NKVD, the KGB's predecessor, on orders from Joseph Stalin.
The KGB has also been accused of killing Ukrainian nationalist hero Stepan Bandera with cyanide gas in Munich in 1959 and Bulgarian dissident Georgi Markov with a poison-tipped umbrella in London in 1978.
A public inquiry in Britain into the death by radiation poisoning of former spy Alexander Litvinenko in London in 2006 concluded earlier this year that the killing was "probably" approved by the head of the KGB's successor agency the FSB.
Litvinenko died after drinking tea laced with radioactive polonium in a London hotel.
The inquiry said it was deliberately administered to him by two Russian contacts he was meeting.
Central Intelligence Agency (CIA) officer Aldrich Ames worked in Turkey, Mexico, Italy and in Soviet counter-intelligence in the United States before being exposed as a mole in 1994.
He began giving information to the Soviet Union in the 1980s but only aroused suspicion later with his cash purchase of a house in Virginia, expensive dental work, a Jaguar car and tailor-made suits.
A report by the Department of Justice's Inspector General concluded his betrayal led to the "catastrophic and unprecedented loss of Soviet intelligence sources" in 1985 and 1986.
Several people working for US intelligence were reportedly executed as a result of his revelations.
A "sleeper cell" network of suspected Russian agents was uncovered in the United States in 2010, with 10 initial arrests.
The network was allegedly set up by Russian's foreign intelligence agency to infiltrate US policymaking circles, though the information they transmitted was said by US officials to be virtually valueless.
They were exchanged at Vienna airport for four Russians convicted of spying for the West.
The suspects included a Spanish-language newspaper columnist and Vladimir and Lidiya Guryev, a married couple operating in the New York financial arena who took on the identities of Richard and Cynthia Murphy.
The most famous deep-cover agent in the group was real estate agent Anna Chapman, an attractive redhead who mingled in Manhattan high society.
She has since launched a fashion line in Russia, worked as a television presenter, offered marriage to US whistleblower Edward Snowden and posed in lingerie for the Russian men's magazine Maxim.