Paul is a historian of modern Europe and of international relations, chiefly in the 20th century and above all since 1945. His core interest is in the political development of Europe over the last century or so, and especially in the impact of Europe’s two most powerful states in that period, Germany and the Soviet Union, on the continent and the wider world. The modern history of Germany and the Soviet Union, together with the history of intelligence and security, are his particular fields of specialism. The history of Communism runs as a thread through all these interests. These core interests have also led Paul to take an interest in states that have felt threatened by Germany and the Soviet Union, such as Britain and the United States. Most of Paul's publications concern either Communist security and intelligence agencies, or British and Western intelligence collection during the Cold War on the German Democratic Republic and the Soviet Union.
Paul is currently researching into the history of the security and intelligence services of the former Communist Bloc, above all the East German Stasi and the Soviet KGB. Previously, he researched into the history of British and other Western intelligence agencies during the Cold War. This research led to his monograph, Spying on Science: Western Intelligence in Divided Germany, 1945-1961 (Oxford University Press, 2006) which examines the proliferation of "weapons of mass destruction" and covert attempts by western governments to undermine the development of such weapons programmes.
He is a Fellow of the Royal Historical Society and a member of the editorial board of the International Journal of Intelligence and CounterIntelligence. In 2008, Paul was for six months a visiting scholar at the Friedrich-Meinecke-Institut of the Free University of Berlin.