Theatre censorship was introduced by statute in 1737 to protect the Prime Minister from the dramatists’ satirical attacks. From that date to 1968, it was governed by the Lord Chamberlain. Theatre censorship was officially justified by the necessity to control morals on the stage. However, it was political by nature. The system was secret and adapted itself to remain invisible and to avoid being abolished. This article presents the unpublished results of research carried out in archival holdings (British Library, National Archives, House of Lords Library, Bodleian, and various private collections) as well as of interviews conducted in 2002-3 with the MPs and dramatists involved in the censorship debates of the 1960s. It first aims at explaining the process of licensing and banning plays through representative examples of religious, political and moral censorship. It also traces the changes which characterized 20th century censorship, thereby accounting for the growing tendency towards permissiveness. Finally, archival material which was recently opened under the thirty-year rule enables us to establish the interlocking relationship between the Lord Chamberlain and the Government.