In a press release Apple announced that the board named Cook as CEO, and that Jobs has been elected Chairman of the Board and Cook will join the board.
Since 2004, Jobs has battled cancer, the effects of a liver transplant and undisclosed illnesses.
Tim Bajarin, president of the consulting firm Creative Strategies, has followed Jobs' career for decades.
"Steve as a person and as a visionary is relatively irreplaceable," Bajarin said. "But at the same time, let's be clear. Over the 7 years especially, he's built up a staff, what we call a deep bench, who not only understand his vision, but understand even more so, something he can't do, how to execute on his vision."
The Wall Street Journal reports that Cook has been considered a leading candidate to succeed Jobs, who has been on medical leave since January. Jobs did appear at the iPad 2 launch in March, and Apple's developers conference in June.
Cook, 50, has been widely considered as the leading candidate to succeed Mr. Jobs, who has been on medical leave since January. The 13-year Apple veteran, who joined the company shortly after Mr. Jobs took over the company for the second time in 1997, has been running the day to day operations during this period as he has done during two prior medical leaves of absence by Mr. Jobs in the last seven years.
Apple's stock was down in after-hours trading.
The press release from Jobs' announcement.
PRESS RELEASE: Letter from Steve Jobs
August 24, 2011–To the Apple Board of Directors and the Apple Community:
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.
I hereby resign as CEO of Apple. I would like to serve, if the Board sees fit, as Chairman of the Board, director and Apple employee.
As far as my successor goes, I strongly recommend that we execute our succession plan and name Tim Cook as CEO of Apple.
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.
I have made some of the best friends of my life at Apple, and I thank you all for the many years of being able to work alongside you.
In 2005, Jobs gave a commencement speech at Stanford University explaining why and how he founded Apple.