In Britain as in other countries security service files have come to provide an important resource for the writing of communist history. This paper discusses some of practical, ethical and methodological challenges they pose for historians. In Britain the selective release of small batches of redacted files was undertaken from the 1990s as part of the post-Cold War rebranding of the MI5 state security service. No meaningful public consultation took place regarding the principles governing the release of these materials. Nor were details made available of the scope of state surveillance or of the individuals and organisations subjected to it. The lack of transparency and accountability was compounded by the asymmetry between observers and observed and the withholding of information regarding the identities, associations and career histories of security operatives. Drawing on published and unpublished examples, the paper characterises this as a project for the ‘securitisation’ of communist and wider left history: one that seeks to validate MI5’s past and continuing role by accentuating issues of espionage while filtering and in many cases destroying evidence of the routine surveillance of political and social movement activism.