Transport accounts for nearly a quarter of carbon dioxide emissions from fuel combustion. The price attached to these emissions is critical to climate policies and emissions mitigation efforts in the sector. As the impact of emissions on climate does not depend on where CO2 is released, the price of carbon should be uniform. In reality, however, it varies immensely, reflecting the complexity of assessing climate impacts. This report reviews the three key challenges in considering the effects of carbon dioxide emissions in economic appraisal: the valuation of carbon dioxide emissions, the treatment of uncertainty in climate change and the approach used to discounting future costs and benefits. The report reviews current approaches in selected countries (France, Germany, Japan, The Netherlands, New Zealand, Norway, Sweden, the United Kingdom and the United States) and provides examples of good practice and recommendations for national and international policy making.