The market value of government debt equals the present discounted value of primary surpluses. Applying present value decompositions from asset pricing to this valuation equation, I find that half of the variation in the market value of debt to GDP ratio corresponds to varying forecasts of future primary surpluses, and half to varying discount rates. Variation in expected growth rates is unimportant. Institutional subscribers to the NBER working paper series, and residents of developing countries may download this paper without additional charge at www.nber.org.