The Department for Transport (DfT) has concluded, based on cost-benefit analysis, that more capacity at the UK's busiest airport will do the country more good than harm. The problems of relying on cost-benefit analysis to make important decisions about social welfare have been well documented in the literature: cost-benefit analysis does not take equity into consideration; costs and benefits to future generations are assumed to be of little consequence to today's decision makers; and ad hoc methods are used to assign money values to priceless environmental and social goods. But in addition to these serious critiques, one of the little-noticed features of the DfT analysis is its absolute reliance on a highly abstract, unmeasurable benefit derived from economic theory: the so-called "consumer surplus" that Heathrow expansion is projected to generate over the next several decades, which is expected to outweigh its (less abstract and more easily measured) costs. The decision of whether or not to expand capacity at Heathrow Airport - by adding a new runway, changing to a noisier but higher-turnover mode of operation, and lifting legal limits on the number of flights - rests on DfT's estimation of consumer surplus, or "generated user benefits." This report assesses the validity of the DfT's assumptions and reliance on consumer surplus theory. DfT's (2007a) UK Air Passenger Demand and CO2 Forecasts predicts that if demand for flights into and out of the UK were not constrained by limited airport capacity, the number of passengers would increase from 228 million passengers per year in 2005 to 460-540 million in 2030. These forecasts include the assumption that starting in 2010 UK airfare will include a carbon tax, to be gradually increased over ten years until it reaches the full cost of each flight's greenhouse gas emissions. DfT (2007a) estimates 4.4-5.2 billion euro in total net benefits (over and above costs) from adding a third runway at Heathrow Airport to open in 20202. The total net benefits of switching Heathrow's existing runways to "mixed mode" operations (that is, allowing both runways to be used for arrivals and departures) from 2010 to 2020, lifting Heathrow's legal capacity limits in 2015, and adding a third runway in 2020 is estimated to be 5.4-6.2 billion euro. Very similar scenarios in the DfT's (2007b) Adding Capacity at Heathrow Airport - Public Consultation have higher costs and benefits, but the same net benefits: 4.4-5.2 billion euro for adding a third runway, and 5.4-6.2 euro billion for mixed mode operations on the existing runways from 2010 to 2020, lifting the legal capacity limits in 2015, and adding a new runway in 2020.