This thesis is a study of the British intelligence assessments produced by the UK's Joint Intelligence Committee regarding the Soviet Union's nuclear capabilities and intentions. It examines the origins of such intelligence, the various organisations that collected, collated and analysed it and how it fed into the Joint Intelligence structure. The thesis seeks both to synthesise existing historical analysis and add new evidence on intelligence organisation, collection, analysis and dissemination by examining the development of such assessments over a twenty-five year period and considering how well they reflected and informed British governments about the status and progress of the Soviet nuclear threat. Lastly, it analyses how this intelligence fed into and may have affected wider British military and ministerial decision-making regarding the course of the UK's nuclear weapons policy between 1945 and 1970.