The Cambridge Spy Ring was a group of spies recruited by the Soviet scout Arnold Deutsch. Using the code name Otto, he was the controller for the Cambridge Five from 1933 to 1937 but also controlled a further 15 spies.
Originally the ring was known as the Cambridge four but another spy was discovered as more evidence came to light. The men that made up the initial four were:
Donald Duart Maclean
This RAF article provides some good information on each of the individuals.
The fifth member, John Cairncross, is regarded as the most effective of the five by the KGB. He successfully penetrated a greater variety of the corridors of power and intelligence.
Cairncross was only discovered when he confessed to spying in 1951.
Although none of these spies were caught during the build up to the Cold War, it is important to realise that espionage was a source of intelligence for the big three. They tried to integrate spies into the governments of other countries. Russia in the case of the Cambridge Five, was very successful. We see espionage as a key segment of Comintern in the build up to the Cold War. Another area that led to tension.