Through a long illness the writer had the opportunity to study the available information concerning stock market behavior. Gradually the wild, senseless and apparently uncontrollable changes in prices from year to year, from month to month, or from day to day, linked themselves into a law-abiding rhythmic pattern of waves. This pattern seems to repeat itself over and over again. With knowledge of this law or phenomenon (that I have called the Wave Principle), it is possible to measure and forecast the various trends and corrections (Minor, Intermediate, Major and even movements of still greater degree) that go to complete a great cycle.