Knowles is a historian of modern cities, technology and public policy–with a particular focus on risk and disaster. His most recent book is The Disaster Experts: Mastering Risk in Modern America (Univ. of Pennsylvania Press, 2011). He is the editor of Imagining Philadelphia: Edmund Bacon and the Future of the City (UPenn Press, 2009); and has published articles, essays and book reviews in The Next American City, Isis, History and Technology, Public Works Management and Policy, Technology and Culture, Business History Review, Enterprise and Society, The Smart Set andAnnals of Science. His opinion pieces have run in The New York Times, The Hill and The Philadelphia Inquirer, and he has been a media commentator for such outlets as TIME.com, Al Jazeera, NPR, FOX News, Inside Edition and CNBC.com.