This Briefing Note discusses how much scope there is to raise revenue from the very rich by increasing income tax rates and assesses in detail the amount of revenue that is likely to be raised by the government's proposed reforms. It extends analysis presented in the 2009 IFS Green Budget and updates some calculations in a submission to the Mirrlees Review. It also discusses information recently released by HM Treasury and HM Revenue & Customs concerning their methodology for calculating how much revenue these reforms will raise. The Briefing Note shows that there is considerable uncertainty over the revenue that could be raised from the very rich by increasing income tax rates, both because we cannot be certain about the distribution of incomes above £100,000 and because we cannot be certain how those affected will respond to the tax increase. It goes on to discuss under what conditions the measures in PBR 2008 could yield as much revenue as the Treasury is forecasting.