My main research field has been the history of Nazi Germany, and most of my publications are in this field, although I've had a subsidiary interest in the history of women and of sexuality, and I have now also moved into other fields of interest. My most recent publications in the field of German history include an edition of a memoir by Gabriele Herz, a German Jewish woman who was detained in an early Nazi concentration camp for women, Moringen, in 1936/7: Gabriele Herz, The Women's Camp in Moringen. A Memoir of Imprisonment in Germany 1936-1937 (New York/Oxford Berghahn Books 2006); a collection of  essays on the history of Nazi Germany in the Oxford University Press the Oxford Short History of Germany series, Nazi Germany (Oxford, OUP 2008); and, with Nikolaus Wachsmann, Concentration Camps in Nazi Germany. The New Histories (London/New York, Routledge 2010).

          My other principal interest is in the history of individual identity and identity documents, and this is the focus of my current research. I began by looking into the history of tattoos as marks of identity; this work has been published in a collection I edited, Written on the Body: The Tattoo in European and American History (Princeton University Press and Reaktion Books, London, 2000). I have also co-edited, with John Torpey, a further collection of essays on many aspects of the history of identification, Documenting Individual Identity : The Development of State Practices in the Modern World (Princeton University Press, 2001).  Since 2008 I have convened IdentiNet, an International Network on the history of individual identity documentation funded by a grant from the Leverhulme Trust, in collaboration with Professor Edward Higgs (University of Essex).

   I am currently working on the proof and policing of invididual identity in Nazi Germany, most recently as a visiting fellow at the Max Planck Institute for the History of Science in Spring 2011. I have been an editor of History Workshop Journal since the 1970s.

          Not long ago someone asked me to describe in a few words what kind of a historian I am - he wanted to know how I would place myself intellectually in what is now a very diverse discipline. My answer was that I thought of myself as an eclectic historian, i.e. not someone who is committed to only one way of thinking and doing the subject. I think this reflects an attempt to integrate all the ways in which a sense of history has influenced me - my childhood fascination with the past because, perplexingly, it was no longer there but somehow still with us; a rather formal 1960s Oxford training in empirical history; the period I spent working with Arnold Toynbee, who was simultaneously a conventional academic, a man of formidably wide historical knowledge, and an intellectual maverick; my exposure over the years to the intellectual changes in the discipline, which have left me with a desire to try to understand new and unfamiliar ideas before passing judgment on them; my involvement in political activism of various kinds, from political parties to trade unions to the feminist and gay movements, backed by a consciousness of the history of popular politics; finally, a scepticism and irreverence for dogmatic excess, which I hope isn't incompatible with a firm commitment to certain principles of justice and humanity. Since I hope that these will be better practiced in the future than they have been in the past, it has always seemed to me to be very instructive to study how (to paraphrase the words of a famous historian) we do make our own history, yet not under conditions of our own choosing but always with the weight of the past upon us. To me this means that if we can understand that past we may also free ourselves from some of its burdens.