Research History provides an essential context for understanding the development of many contemporary problems. Without an understanding of such historical background, many attempts at contemporary problem solving are doomed to failure. Professor Tarr is a historian whose research focuses on the history of urban technologies and urban infrastructure systems, as well as the development of environmental problems and policy. More specifically, he has written about the effects of transportation innovations, the uses of the telegraph in the urban context, and the development and impacts of water supply and waste water systems. Professor Tarr's environmental work has dealt also with air, water, and land pollution, and the cross-media problems created by technological choices and changing disposal practices. In addition, Professor Tarr has examined problems of industrial pollution. Professor Tarr has also written about environmental policy formation on the local, state, and federal levels, and the roles of various professional groups in setting priorities. This research has dealt primarily with changing conceptions of risk in the face of new knowledge and new technologies, as well as societal value change. Much of his environmental and technology-related research has been done in collaboration with engineers. His most recent research deals with the environmental Impacts of Conventional Natural Gas Drilling in Pennsylvania.