Steve Jobs has resigned as chief executive officer of Apple, Inc., for the second time since he co-founded the company in 1976.

He has been elected chairman of Apple's Board of Directors, effective immediately. Jobs is also a member of the board of directors of Pixar, where he was CEO for ten years prior to Disney's acquisition of the company in 2006.

Tim Cook, formerly Apple's chief operating officer, has been appointed CEO by Apple's board.

Jobs has published a valedictory letter that only alludes to the health issues that appear to have motivated his decision to step down.

"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," Jobs wrote. "Unfortunately, that day has come."

Apple did not immediately respond to a request to confirm that Jobs' health motivated his decision.

In 2004, Jobs underwent an operation to treat a rare form of pancreatic cancer. Though the operation was regarded a success, Jobs has continued to confront health issues in the years since then. He took a six-month leave of absence in January, 2009, and was granted another leave of absence this year.

Speaking on behalf of Apple's board, Art Levinson, chairman of Genentech, lauded Jobs's accomplishments and leadership. "Steve's extraordinary vision and leadership saved Apple and guided it to its position as the world’s most innovative and valuable technology company," he said in a statement.

Jobs first resigned from Apple in 1984 following a boardroom power struggle and went on to found Next Computer. Apple acquired Next in 1996, bringing Jobs back to the company. Jobs took over as CEO the following year and began laying the groundwork for Apple's transformation into one of the most respected and successful companies in the world.

Across the Internet, fans and competitors expressed sadness at the presumed state of Jobs' health and admiration for his accomplishments. Investors expressed their concern by selling Apple stock, sending it down sharply in after-hours trading.

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