Ari Rabl obtained his PhD in physics at the University of California, Berkeley and worked for many years as research scientist for energy technologies at Argonne, NREL and Princeton University. He moved to the École des Mines de Paris as Senior Scientist at the Centre Energétique et Procédés until his retirement in 2007, and now continues to work as a consultant. For the last 20 years, his work has focused on environmental impacts and costs of pollution. He is one of the principal participants of the ExternE Project series (External Costs of Energy) of the European Commission DG Research, for which he has co-ordinated several research projects. Joseph V. Spadaro is Research Professor at the Basque Centre for Climate Change, Bilbao, Spain where he works on low carbon planning strategies, and climate change and health vulnerability. He is also Environmental Systems Engineer at Argonne National Labs, Decision and Information Sciences Division, USA, working on environmental impact assessment of energy technologies. He has been a member of the core team of principal investigators in the ExternE Project series (European Commission, DG Research) since the mid-1990s, and has acted as expert consultant for international organizations, national governments and private industry in projects related to impacts and costs of pollution. Mike Holland is a freelance consultant based in the UK. He has worked on assessment of pollution effects on ecosystems and health since 1986, and he has performed a large number of cost-benefit analyses for national government and the European Commission, particularly in relation to measures to improve air quality. He is currently the chair of NEBEI (Network of Experts on Benefits and Economic Instruments) under the UNECE Convention on Long Range Transboundary Air Pollution.