CEO Steve Jobs at an Apple event in October to introduce Mac OS X Lion.

Apple's Steve Jobs will be taking a medical leave of absence for the second time in two years but will remain CEO of the company, involved in strategic decision-making.

Chief Operating Officer Tim Cook will assume responsibility for the company's day-to-day operations, amid continuing investor concerns over Apple's plans for eventual transition in the corner office.

Some notable moments for Jobs and his companies over the years.

1976 Co-founds Apple with Steve Wozniak.

1984 Apple introduces the Mac.

1985 CEO John Sculley engineers Jobs' ouster from Apple.

1985 Founds Next Computer (later becomes Next Software).

1986 Buys computer graphics division of LucasFilm, to be renamed Pixar.

1996 Returns to Apple as adviser when Apple buys Next Software

1997 Named interim CEO of Apple.

2001 Apple introduces iTunes software and the iPod' the iTunes store follows in 2003.

2005 Disney buys Pixar' Jobs becomes largest shareholder.

2007 Apple introduces the iPhone.

2009 Takes 6-month leave of absence for medical reasons.

2010 Apple introduces the iPad.

January 2011 Takes medical leave of absence.

The stock market is closed today for a federal holiday. Apple is scheduled to report its first-quarter fiscal 2011 earnings tomorrow after 1 p.m. PT.

Jobs, a pancreatic cancer survivor, took a leave in the first half of 2009 to deal with issues related to his medical treatments. In announcing that hiatus, he said he would step away for six months, and he returned to full-time work on schedule.

This time around, the 55-year-old Jobs did not give a time frame for his return. Here's the text of his e-mail to employees, provided by Apple this morning:

At my request, the board of directors has granted me a medical leave of absence so I can focus on my health. I will continue as CEO and be involved in major strategic decisions for the company.

I have asked Tim Cook to be responsible for all of Apple's day to day operations. I have great confidence that Tim and the rest of the executive management team will do a terrific job executing the exciting plans we have in place for 2011.

I love Apple so much and hope to be back as soon as I can. In the meantime, my family and I would deeply appreciate respect for our privacy.

Steve

In January 2009, Jobs said that he was suffering from a hormone imbalance that was impeding his body's ability to absorb certain proteins. In April of that year, Jobs underwent liver transplant surgery, and he returned to work by early July. Cook was the one to step in to run the day-to-day operations of the company during that leave as well, guiding the company through the launch of the iPhone 3GS, as well as some of the final stages of the production of the iPad.

In the months preceding that announcement two years ago, Jobs' health had been the topic of intense speculation. In an appearance at the company's 2008 Worldwide Developers Conference, Jobs appeared to have lost a considerable of weight. Apple and Jobs rebuffed any questions on the matter, calling his health a "private matter."

No such speculation led up to today's announcement. Jobs has made regular appearances in support of new Apple products, most recently at an October event introducing the latest MacBook Air and Mac OS X Lion. He was supposed to appear at an event in San Francisco this week with Rupert Murdoch to announce the launch of The Daily, an iPad-only newspaper, but the event was rescheduled late last week.

Though his immediate health was no longer a major focal point with the press or the public, the topic of who will take over for Jobs at Apple remains a hot one with investors. Many investors feel that Jobs can never truly be replaced as CEO, since his vision and persona are so fundamentally linked with the company, but Apple has been criticized for not publicly grooming a successor.

Apple COO Tim Cook at the company&39's October event

Just recently it came to light that Apple's board of directors had fielded a stockholder request that Apple make its succession plan public knowledge. Apple refused, saying that privately there is an established a plan in place and that disclosing it to the public and competitors would damage the company's ability to retain and recruit top executive talent.

From the mid-1980s to the mid-1990s, Jobs was absent from Apple after a conflict with the board over leadership style. Since his return to Apple in 1997, Jobs has rebuilt the company he co-founded with Steve Wozniak, taking it from near bankruptcy to the most influential consumer electronics company in the world. With more than $50 billion in the bank, Apple now has the second-highest market capitalization in the world, behind ExxonMobil. Beyond the Mac lineup of desktops and laptops, Jobs has transitioned the company into the entertainment business with iTunes and the iPod, and into a leading mobile phone maker with the iPhone. Last year, Apple introduced the touch-screen iPad, which has created a third mobile device category and a success that all of Apple's competitors have recently flocked to try to reproduce.

Cook capably handled the reins during Jobs' 2009 leave of absence. While Jobs was away, Apple delivered two back-to-back blow-out quarters, as well as an updated iPhone OS and new iPhone hardware. Under Cook, Apple also put the finishing touches on the company's new iPods, MacBooks, and Mac OS, which were released shortly after Jobs' return to the helm in late June.

He was rewarded handsomely for keeping Apple on track, getting a bonus of $5 million plus 75,000 stock options for his work filling in for Jobs. He is Jobs' most trusted executive, handling all earnings calls with Wall Street investors and analysts for the last few years, and has had an increasingly larger role at major product introductions. Cook most recently appeared on behalf of Apple last week alongside Verizon executives in New York City to announce the first iPhone that works on Verizon's network.

Cook, 50, joined Apple in 1998 and was promoted to COO in 2004. It was at Cook's behest that Apple pulled out of building its own products and moved to contract manufacturers like Foxconn and others, reducing the company's inventory and drastically improving its margins.

Editors' note: This story was updated several times with background and additional information, most recently at 8:10 a.m. PT.

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Jonathan Skillings is managing editor of CNET News, based in the Boston bureau. He's been with CNET since 2000, after a decade in tech journalism at the IDG News Service, PC Week, and an AS/400 magazine. He's also been a soldier and a schoolteacher. E-mail Jon.

If you have a question or comment for Erica Ogg, you can submit it here. However, because our editors and writers receive hundreds of requests, we cannot tell you when you may receive a response.

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Erica Ogg is a CNET News reporter who covers Apple, HP, Dell, and other PC makers, as well as the consumer electronics industry. She's also one of the hosts of CNET News' Daily Podcast. In her non-work life, she's a history geek, a loyal Dodgers fan, and a mac-and-cheese connoisseur.