You can learn more about this history of MI5 at The National Archives in Kew. We have released nearly 4,000 files to The National Archives, covering a period up to 1957.
The National Archives website provides a full overview of the most noteworthy files released from April 2000 onwards.
All of the released files can be examined by National Archives pass holders, free of cost, and ordered through the National Archives catalogue. Some of the files can also be seen online through the Discovery opens in a new window service.
We usually release files twice a year, generally each spring and autumn. See What's in The National Archives? for a brief overview of the topic areas covered by our files. It should be noted that some of the files we release are redacted to ensure the removal of sensitive information, and some individual documents may be retained for the same reason (see Retention and destruction of files for more on our document release and retention policies).