Unlike many organisations mentioned on this website, M.I.6. is a real organisation that was formed in 1909. M.I.6. is an abbreviation for Secret Intelligence Service. M.I.6. was formed in 1909 as part of the Secret Service Bureau when the threat coming from Germany was becoming very high. It was consisted of a number of army personnel who would track Germans in the country. As their roles became more and more diverse, the organisation split into two, one to deal with internal threats (Security Service better known as Military Intelligence Section 5) and external threats (Secret Intelligence Service also known as Military Intelligence Section 6).
In the films and books, the head of the organisation is 'M' but in real life, the head has the letter designation of 'C'. The 'C' comes from the first head of M.I.6., Captain George Mansfield Smith-Cummings who used to write the letter 'C' in green pen on papers that he wished to read. From that moment on, the head of the organisation is known as 'C' and writes in green pen. The sister organisation, M.I.5, has the letter designation of 'K' after the first and longest chief, Vernon Kell.
For years, the two organisations existence were denied by the British Government until the 1994 with the Intelligence Services Act. Until it was officially recognised, James Bond was a Commander in Naval Intelligence. Bond still has a background in Naval Intelligence but he is more closely associated with M.I.6. now.
M.I.6. has a close working relationship with other agencies around the world including that of the C.I.A., the main foreign threat intelligence service of the United States of America. It will also have a working relationship with the F.S.B. which stands for Federal'naya sluzhba bezopasnosti Rossiyskoy Federatsii. In English, the FSB stands for Federal Security Service of the Russian Federation. How deep the F.S.B. and M.I.6.'s working relationship is is unknown. Speculating, it would be limited to terrorist information sharing, defence, nooooooo.
The headquarters of M.I.6. is Vauxhall Cross, South London, for one of the most secretive organisations in the U.K., its building is one of the conspicuous buildings in the United Kingdom and is recognisable by many people. Its also been seen in numerous spy films including James Bond films.
The most famous intelligence officer, (they don't call themselves secret agents, to them agents are the people they tap for information) is James Bond, 007, licenced to kill. As to whether they have licences to kill is open to speculation. The official story line of the British Government is that they don't but people point to so-called James Bond clauses in Government Acts of Parliament. 1
Unlike James Bond, M.I.6.'s only adventures allegedly into space is sending spy satiellites up to provide intelligence for the people on the ground.