Apple CEO Steve Jobs has resigned and will be replaced by former Chief Operating Officer Tim Cook, the company said late Wednesday. Jobs will stay on as Apple’s chairman.

Apple made no mention of Jobs’ health in its statement about the change, but Jobs alluded to it in the letter of resignation he sent to Apple’s board on Wednesday and later released publicly.

“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,” wrote Jobs, who has been on medical leave since January. Cook has been filling in as the company’s leader.

Apple’s board took pains to praise Jobs, who lead a historic turnaround for the once-ailing company. Apple shares were briefly halted in after-hours trading as Apple announced its leadership change. When trading resumed, shares dropped 5 percent.

“Steve’s extraordinary vision and leadership saved Apple and guided it to its position as the world’s most innovative and valuable technology company,” board member Art Levinson said. “In his new role as chairman of the board, Steve will continue to serve Apple with his unique insights, creativity and inspiration.”

In January, Jobs said he would take another medical leave of absence, two years after a six-month sabbatical during which he received a liver transplant.

Jobs made his last public appearance in June when he unveiled iCloud, a free wireless storage and syncing service, at Apple’s annual Worldwide Developers Conference. Jobs received a standing ovation when he took the stage.

Jobs co-founded Apple in 1976 from his family’s garage with Steve Wozniak. Nine years later, Jobs parted ways with Apple after disagreements with management. He returned as a consultant in 1996 and became interim CEO a year later. In 2000, he took the jobs permanently.

Since then, Apple has pulled off a Phoenix-like resurrection and become the world’s most valuable tech company.

“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,” Jobs wrote in his resignation letter. “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.”

Six years ago, Steve Jobs delivered his only commencement speech — one that is often cited as the speech of his life. “Your time is limited, so don’t waste it living someone else’s life,” he told the crowd.

It was a powerful talk, given the CEO’s history of illness. Jobs was diagnosed with a treatable form of pancreatic cancer in 2004. Since then, both his health and how the company would run without him have been topics of intense speculation.