Keith Hartley is Professor of Economics and Director for the Centre for Defence Economics at the University of York. He is the author of many books and articles, including NATO Arms Co-operation: a Study in Economics and Politics (1983). Todd Sandler is Professor of Economics and Political Science at Iowa State University. He has written many books and articles, including The Theory of Externalities, Public Goods, and Club Goods (with Richard Cornes, 1986). Both Hartley and Sandler are Editors of Defence Economics.