A specialist in Islamic religious thought and in the history and theology of Christian-Muslim relations, his recent studies focus on the character of arguments used against Christian doctrines in works of Muslim systematic theology. He is Managing Editor of the book series The History of Christian-Muslim Relations, published by Brill of Leiden, and Chief Editor of the journal Islam and Christian-Muslim Relations.
Since 2006 he has led Christian-Muslim Relations, a Bibliographical History, a project to bring to light all the known works written by Muslims and Christians about one another and against one another. The first part of the project, CMR600, produced five encyclopaedic volumes that covered the period 600-1500. The second part, CMR1900, continues and completes this up to the year 1914.