Ten years ago, most textbooks on financial management advocated the thesis that stock returns are essentially unpredictable. This theory is called the Random Walk Approach to the development of asset prices. The approach said that the stock market is subject to random changes, which are, by definition, unpredictable. Apparent predictabilities, if ever discovered, were either dismissed as statistical artifacts or as data that cannot be exploited after transaction costs. In the meantime, the world of financial economics has turned upside down. We now realize clearly that returns are indeed...