My general field of study is Early Christian, Byzantine, and Early Medieval architecture and archaeology. My main fields of research are the architecture and archaelogy of Byzantine Constantinople where I surveyed and studied two former churches, the Myrelaion and Kalenderhane Camii. I was also archaeological consultant for the Istanbul subway and Bosphorus tunnel project and for a study of the structure of Hagia Sophia. I was also the first to use dendrochronology (tree-ring dating) for the study of Byzantine architecture, working in Turkey, Greece, the former Yugoslavia, and Albania). Recently I completed an eight-year tree-ring survey of the Heptapyrgion Castle in Thessaloniki, Greece. I have been active in the organization and administration of American overseas research centers. I was a founder of the Council of American Overseas Research Centers, and have close affiliation with the American Research Institute in Turkey. I formally retired from the University of Pennsylvania in 2002 in order to devote full time to research and publication.