This article analyzes the implications of money illusion for investor behavior and asset prices in a securities market economy with inflationary fluctuations. We provide a belief-based formulation of money illusion which accounts for the systematic mistakes in evaluating real and nominal quantities. The impact of money illusion on security prices and their dynamics is demonstrated to be considerable even though its welfare cost on investors is small in typical environments. A money-illusioned investor's real consumption is shown to generally depend on the price level, and specifically to decrease in the price level. A general-equilibrium analysis in the presence of money illusion generates implications that are consistent with several empirical regularities. In particular, the real bond yields and dividend price ratios are positively related to expected inflation, the real short rate is negatively correlated with realized inflation, and money illusion may induce predictability and excess volatility in stock returns. The basic analysis is generalized to incorporate heterogeneous investors with differing degrees of illusion.