A matter of intelligence concerns the surveillance of anti-Nazi German refugees during the 1930s and 1940s by the British security service MI5. When Hitler took power in 1933, the Nazis began a reign of terror against their political opponents: communists, socialists, pacifists and liberals, many of whom were forced to flee Germany. Some of these political refugees came to Britain, where MI5 kept them under close surveillance. This study is based on the personal and organisational files that MI5 kept on them during the 1930s and 1940s - or at least those that have been released to the National Archives - making it equally a study of the political refugees themselves. Although this surveillance exercise formed an important part of MI5's work during that period, it is a part that it seems to have disowned or at any rate forgotten: the recent official history of MI5 does not even mention it, nor do its 'unofficial' counterparts. This study therefore fills a considerable gap in historical research. It traces the development of MI5 surveillance of German-speaking refugees through the case files of some of its individual targets and of the main refugee organisations; it also considers the refugees' British supporters and the informants within the refugee community who spied on fellow-refugees, as well as MI5's tussles with the Home Office and other official bodies. Finally, it assesses how successful - or how useful - this hidden surveillance exercise actually was. The book will appeal to academics in the fields of history, politics, intelligence studies, Jewish studies, German studies and migration studies; but it is accessible to the general reader interested in Britain before, during and after the Second World War.