Tim Cook is stepping down as CEO. In nearly 15 years as chief executive, Cook turned Apple into a global powerhouse, building on the legacy of his legendary predecessor Steve Jobs.Why it matters: Cook, who oversaw the launch of the Apple Watch and AirPods, will be replaced by hardware expert John Ternus.The big picture: Apple is one of the most valuable companies in the world.Driving the news: Cook will become executive chairman of Apple on Sept. 1, the company said Monday.Ternus, senior VP of hardware engineering, has been widely viewed as Cook's likely successor.What they're saying: "It has been the greatest privilege of my life to be the CEO of Apple and to have been trusted to lead such an extraordinary company," Cook said in a statement.Editor's note: This story is breaking and will be updated.