The real interest rate plays a central role in many important financial and macroeconomic models, including the consumption-based asset pricing model, neoclassical growth model, and models of the monetary transmission mechanism. The authors selectively survey the empirical literature that examines the time-series properties of real interest rates. A key stylized fact is that postwar real interest rates exhibit substantial persistence, shown by extended periods when the real interest rate is substantially above or below the sample mean. The finding of persistence in real interest rates is pervasive, appearing in a variety of guises in the literature. The authors discuss the implications of persistence for theoretical models, illustrate existing findings with updated data, and highlight areas for future research. ( ines its long-run properties. This paper selectively reviews this literature, highlights its central findings, and analyzes their implications for theory. We illustrate our study with new empirical results based on U.S. data. Two themes emerge from our review: (i) Real rates are very persistent, much more so than consumption growth; and (ii) researchers should seriously explore the causes of this persistence.First, empirical studies find that real interest rates exhibit substantial persistence, shown by extended periods when postwar real interest rates are substantially above or below the sample mean. Researchers characterize this feature of the data with several types of models. One group of studies uses unit root and cointegration tests to analyze whether shocks permanently affect the real interest rate-that is, whether the real rate behaves like a random walk. Such studies often report evidence T he real interest rate-an interest rate adjusted for either realized or expected inflation-is the relative price of consuming now rather than later. 1 As such, it is a key variable in important theoretical models in finance and macroeconomics, such as the consumption-based asset pricing model (Lucas, 1978; Breeden, 1979; Singleton, 1982, 1983), neoclassical growth model (Cass, 1965;Koopmans, 1965), models of central bank policy (Taylor, 1993), and numerous models of the monetary transmission mechanism.The theoretical importance of the real interest rate has generated a sizable literature that exam-1 Heterogeneous agents face different real interest rates, depending on horizon, credit risk, and other factors. And inflation rates are not unique, of course.