William J. O'Neil was one of the most influential figures in modern growth investing. At 30 he became one of the youngest people ever to buy a seat on the New York Stock Exchange, and in 1963 he founded William O'Neil & Co., pioneering computerized stock databases for institutions.

In 1984 he launched Investor's Business Daily as a data-driven rival to the financial press. But his lasting legacy is CAN SLIM, the growth-stock framework built from studying the biggest winning stocks in market history, and popularized in his classic How to Make Money in Stocks, which has sold millions of copies.

O'Neil's core insight ran against instinct: the strongest stocks often look "too high" precisely when they are ready to run. He died on May 28, 2023, at age 90, leaving a methodology still taught and traded worldwide.