There are many theories regarding the fluctuations and patterns of the stock market and the best ways to invest money. Malkiel is a proponent of the Efficient Market Hypothesis, or Theory, known as EMH or EMT, which asserts that the stock market reflects all relevant information immediately. This assertion means that there is no gap period between information becoming relevant and the corresponding change in the price of a stock. As an example, if a pharmaceutical company develops a new drug that will likely sell well and increase the earnings of the company, that corresponding increase in the price of their stock will happen as soon as that information is released, not days after the announcement or even when the drug is actually released. EMH is a debatable theory, as it concludes that no one can consistently beat the market, which forms the basis of Malkiel’s argument in Random Walk.

Malkiel presents two other theories of investing in Random Walk: The firm-foundation theory, which asserts that stocks have an accurate price that the market will either overvalue or undervalue; and the castle-in-the-air theory, which relies on the psychology of investors to determine which stocks are going to rise in value.