The 25 richest self-made billionaires

Net worth: $18.7 billion

Age: 79

Country: US

Industry: Diversified investments

Source of wealth: Self-made; Icahn Enterprises

Carl Icahn has made a lifelong habit and lucrative career out of agitating undervalued and poorly managed companies to change their ways. Since founding his own investment firm in 1968, Icahn has become one of the most powerful people in finance, investing in scores of high-profile companies, including RJR Nabisco, Philips Petroleum, Viacom, Marvel, Time Warner, Netflix, and Herbalife.

And he usually gets his way. When Icahn revealed that he held a stake in Apple worth more than $1 billion in 2013, the company’s stock shot up and CEO Tim Cook responded to Icahn’s critique of the company. Cook has even come around to some of Icahn’s views, and Icahn said in the fall that he may buy an even larger stake.

Now Icahn’s leading the charge against AIG, publishing an open letter in January calling for the insurance giant to break up into a “smaller, simpler company.”

Icahn has said that he has no plans of retiring from pestering corporate executives, but a career shift may nonetheless be in the works. Donald Trump has said that if elected US president, he would bring Icahn in as Treasury secretary — a position which Icahn said he would accept despite some disagreements with the combative real-estate mogul’s positions.