Apple said on Wednesday that Steven P. Jobs, its co-founder and CEO, would step down. Tim Cook, the chief operating officer, will take over the position.
In a letter sent to Apple's board of directors and "the Apple community," Mr. Jobs said he would like to remain as chairman of the board and an Apple employee.
(Text of the resignation letter)"I have always said if there ever came a day when I could no longer meet my duties and expectations as Apple's CEO, I would be the first to let you know," Mr. Jobs wrote. "Unfortunately, that day has come."
Mr. Jobs, 56, had surgery for pancreatic cancer in 2004 and had a liver transplant in early 2009. His health has gone through considerable ups and downs in recent years, according to a person briefed on his condition.
Although he has officially been on medical leave since January, Mr. Jobs has appeared in public to announce new Apple products. In June he was on stage in San Francisco to talk about iCloud, Apple's latest foray into cloud-based computing. It is not clear how much Mr. Jobs has been involved in the company's day-to-day operations recently. Apple is expected to unveil a new model of its hugely successful iPhone this fall.
"I believe Apple's brightest and most innovative days are ahead of it. And I look forward to watching and contributing to its success in a new role," Mr. Jobs said in the letter.
Apple's stock dropped in after-hours trading, falling $16.61, or 4.4 percent, to $359.57.
Mr. Jobs founded Apple in 1976 with Steve Wozniak, and built the company's reputation with the Apple II and Macintosh computers. He left Apple in 1985 after a conflict with John Sculley, then the chief executive. The following year, with a small group of Apple employees, he founded NeXt Computer, which ultimately focused on the corporate computing market, without notable success. In 1986, he bought the computer graphics division of Lucasfilm and re-established it as the independent animation studio Pixar.
A decade later he sold the NeXt operating system to Apple and returned to the company. In short order he was again at the helm and set out to modernize the company's computers.
With the more recent introduction of the iPod, iTunes, iPhone and iPad, Apple has shaken up a range of industries. Mr. Jobs has been awarded much of the credit for turning Apple into one of the world's most valuable companies. Last month Apple briefly became the most valuable company in the world in terms of market capitalization.