Steve Jobs, co-founder and two-time CEO of Apple, offered his resignation to the company's board Wednesday.

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

Jobs has been in poor health for some time. In January, he announced that he would be taking a medical leave of absence from the company. He returned to the public spotlight in March to help launch the iPad 2.

Jobs advised the board to "execute our succession plan" — by naming Tim Cook to replace him. Cook, formerly the COO of the company, has been standing in for Jobs since January and was widely tipped to be his successor. Cook also stood in for Jobs during his bout with pancreatic cancer in 2004. "The Board has complete confidence that Tim is the right person to be our next CEO,” said board member Art Levinson, chairman of biotech firm Genentech, in a prepared statement.

Jobs will not be leaving the company altogether, and the move was clearly well-planned in advance. He has been elected chairman of the board, Apple said Wednesday, and Cook will be joining the board, effective immediately.

"I believe Apple’s brightest and most innovative days are ahead of it," Jobs wrote in his resignation letter to the Apple board. "I look forward to watching and contributing to its success in a new role."

Apple stock was down 7% in after-hours trading but rallied and was down just 5% by 7:50 p.m. ET — suggesting Apple's succession plan had calmed the markets.

Here's a look at some of the most iconic moments from Steve Jobs's career at Apple: