(MONEY Magazine) – John Bogle was an idealistic Princeton economics major in 1951 when he wrote a thesis on the performance of a handful of actively managed mutual funds. His argument: It is almost impossible to consistently beat the market. That idea became the basis of Bogle's investment philosophy and his belief that the fund industry fails most investors. In 1975 he got the chance to put his idea into practice, creating the world's first retail index fund.

Today Vanguard is the second-largest fund company in the world, with $720 billion under management. Bogle no longer runs things day-to-day, but he remains an advocate for industry reform. "The only person more idealistic than the kid who wrote that thesis at Princeton," he says, "is that kid as he approaches his 75th birthday." --C.W.