If Jerry Yang, the chief executive of Yahoo, wasn't feeling hot under the collar already he surely will now that Carl Icahn, one of the world's most aggressive activist investors, has set his sights on the internet company.

This week Icahn waded into the aftermath of Microsoft's recently aborted takeover battle for Yahoo, declaring a $1.5 billion (£773 million) stake and a plan to replace each of its ten board members with his own hand-picked slate. And what Icahn wants, he usually gets.

In a stinging letter to the Yahoo board, Icahn described Yang's flat rejection of the Microsoft proposal as "irresponsible" and "unconscionable" - views he knows are shared by many more big investors seeking to oust Yang from the chief executive's seat.

Since he dropped out of medical school in 1960, the native of Queens, New York, has never shied away from a good fight.

After just seven years cutting his teeth on Wall Street, Icahn founded Icahn & Company, a firm that dealt solely in the shark-infested waters of risk arbitrage and options trading.

Soon after, the young and unknown Icahn began amassing big stakes in some of America's biggest and most powerful corporations intent on stripping out dead wood in the boardroom and on the balance sheet.

One of his early targets was RJR Nabisco, then one of the world's biggest food and tobacco companies. A couple of years later and RJR was bought out for $25 billion by Kohlberg Kravis Roberts, the biggest private-equity deal in history at the time. Those in the know still credit Icahn with starting the ball rolling on that one.

He earned his stripes buying and breaking up TWA, the bankrupt American airline. More recently Icahn forced the takeover of failing software company BAE Systems by Oracle after a feisty battle that went on for months and he is credited with getting Motorola to spin off its mobile phones arm, after he threatened to get rid of the board.

In February 2006 Icahn teamed up with his old friend "Bid 'em Up" Bruce Wasserstein, chief executive of Lazard Freres, the boutique investment bank, to launch an all-out attack on Time Warner. Amid a frenetic media circus, Icahn unveiled a 343-page detailed report into everything he thought was wrong with the cumbersome media conglomerate.

He declared the company should be broken up into four parts after buying back some $20 billion of its own shares. Just ten days later Dick Parsons, the Time Warner chief, buckled and agreed to the buyback plan, to replace his independent directors, to cut costs by $1 billion by the following year and to stay in close contact with Icahn and his advisers.

The way he launched the Time Warner proposal - and indeed that at Motorola and Yahoo - reveals a lot about Icahn's methods and expectations. After first buying up a significant and sizeable stake in a target company, Icahn comes out of the gate with all guns blazing. Usually he will make public a letter to the board, with all kinds of unkind adjectives suggestive of their lack of management skills and nonchalant view of investor relations.

The letter will also often call for drastic and hostile solutions like selling the company, breaking it up and selling off the parts or sacking the entire board. At the very least he will demand a big share buyback.

Icahn was so incensed when he wrote to Motorola's board that he kept switching to capital letters to register his anger. "MOTOROLA HAS STUMBLED AND STUMBLED BADLY," he wrote. And, as Motorola discovered, Icahn does not give up lightly. He was rebuffed time and again by the technology group but Icahn stuck to his guns for more than a year. Finally, whether he gets his way or not, Icahn usually finishes a fight in such a strong position because of the size of the stake he has managed to amass that he gets a seat on the board, better to watch over his new baby.

His record speaks for itself. Icahn has launched and resolved 15 proxy contests in the past eight years. He was granted a seat on the board by six of his targets while four of them agreed to adopt at least some of his demands. The remaining five were dropped. That's a success rate of 66.6%.

Microsoft has given no indication that it will ever come back to the table with a version of its $47.5 billion takeover bid for Yahoo!

But with Icahn in the driving seat, the odds of a major deal being struck in the coming months have risen dramatically.