FIRST ARTICULATED by scholars of the ISalamanca school in sixteenth century Spain,1 purchasing power parity (PPP) is the disarmingly simple empirical proposition that, once converted to a common currency, national price levels should be equal. The basic idea is that if goods market arbitrage enforces broad parity in prices across a sufficient range of individual goods (the law of one price), then there should also be a high correlation in aggregate price levels. While few empirically literate economists take PPP seriously as a short-term proposition, most instinctively believe in some variant of purchasing power parity as an anchor for long-run real exchange rates. Warm, fuzzy feelings about PPP are not, of course, a substitute for hard evidence. There is today an enormous and evergrowing empirical literature on PPP, one that has arrived at a surprising degree of consensus on a couple of basic facts. First, at long last, a number of recent studies have weighed in with fairly persuasive evidence that real exchange rates (nominal exchange rates adjusted for differences in national price levels) tend toward purchasing power parity in the very long run. Consensus estimates suggest, however, that the speed of convergence to PPP is extremely slow; deviations appear to damp out at a rate of roughly 15 percent per year. Second, short-run deviations from PPP are large and volatile. Indeed, the one-month conditional volatility of real exchange rates (the volatility of deviations from PPP) is of the same order of magnitude as the conditional volatility of nominal exchange rates. Price differential volatility is surprisingly large even when one confines attention to relatively homogenous classes of highly traded goods. The purchasing power parity puzzle then is this: How can one reconcile the enormous short-term volatility of real exchange rates with the extremely slow rate at which shocks appear to damp out? Most explanations of short-term exchange rate volatility point to financial factors such as changes in portfolio preferences, short-term asset price bubbles, and monetary shocks (see, for example, Maurice Obstfeld and Rogoff forthcoming). Such shocks can have substantial effects on the real economy in the presence of sticky nominal wages and prices. I See Lawrence H. Officer (1982, ch. 3) for an extensive discussion of the origins of PPP theory; see also Dornbusch (1987).

Editor's Note The Journal of Economic Literature (JEL) regularly reviews books of interest to the economics profession. The Economic Report of the President (ERP) falls under that purview. I have asked a handful of very prominent economists to review the 2006 ERP. Reviewers were chosen to reflect expertise on what I guessed would be key issues. The ERP in principle should provide an accurate assessment of the consensus professional views of economists on any given issue, based on the research to date. Reviewers were asked to evaluate whether the discussion in the ERP in fact accurately summarizes what we as economists know? Reviewers were given free rein over what material they would review in the ERP but were urged to focus on their areas of particular expertise. In the reviews that follow, Martin Feldstein reviews the overview chapter as well as topics relating to macroeconomics. Alan Auerbach reviews the ERP's discussion of tax-related issues, while Ken Rogoff reviews the ERP's discussion of international economic topics. Rebecca Blank writes on labor market issues in the ERP, and Michael Katz reviews the ERP's discussion of health care issues. Many thanks to the reviewers for the quick turnaround.