Jobs, who fought and survived a rare form of pancreatic cancer and revolutionized the technology arena with the iPhone in 2007, is deemed the heart and soul of a company that started in a garage and is today neck-and-neck with Exxon Mobil in the race to become the largest U.S. corporation.
Analysts do not expect Jobs' resignation -- which was more a question of when than if -- to derail Apple's fabled product-launch roadmap, including possibly a new iPhone in September a third iteration of the iPad tablet in 2012.
But the company's shares still dived as much as 7 percent in after-hours trade after the industry icon, who has been on medical leave for an undisclosed condition since Jan. 17, announced he will be replaced by COO and heir apparent Cook.
"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. Unfortunately, that day has come," he said in a brief letter announcing his resignation.
The 55-year-old CEO had briefly emerged from his medical leave in March to unveil the latest version of the iPad and later to attend a dinner hosted by President Barack Obama for technology leaders in Silicon Valley.
Jobs' often-gaunt appearance has sparked questions about his health and his ability to continue at Apple.
"I will say to investors: don't panic and remain calm, it's the right thing to do. Steve will be chairman and Cook is CEO," said BGC Financial analyst Colin Gillis.
AGAIN, DEEP BENCH
Jobs' resignation marks the end of an era at Apple.
A college dropout, a Buddhist and a son of adoptive parents, he started Apple Computer with friend Steve Wozniak in the Jobs family garage in Silicon Valley in the late 1970s.
The company soon introduced the Apple 1 computer. But it was the Apple II that became a huge success and gave Apple its position as a critical player in the then-nascent PC industry, culminating in a 1980 IPO that made Jobs a multimillionaire.
Despite the subsequent success of the Mac, Jobs' relationship with internal management soured, and in 1985 the board removed most of his powers and he left the company, selling all but one share of his Apple holdings.
Apple's fortunes waned after that. However, its purchase of NeXT -- the computer company Jobs founded after leaving Apple -- in 1997 brought him back into the fold. Later that year, he became interim CEO and in 2000, the company dropped "interim" from his title.
On Wednesday, Apple shares slid to $357.40 in extended trading after a brief halt. They had gained 0.7 percent to close at $376.18 on the Nasdaq.
Analysts again expressed confidence in the Apple bench, headed by longtime company No. 2 and supply-chain maven Cook.
"Investors are very comfortable with Tim Cook even though Jobs has been a driver of innovation and clearly an Apple success. Tim has shown Apple can still outperform extremely well when he's been acting as CEO," said Cross Research analyst Shannon Cross.
"I don't know if it's a health issue. I don't know if it is a shock. Most likely it was going to happen at some point. Why today versus another day? I don't know."
