Tim Cook just ended his run as Apple CEO today. Here's what you need to know. Cook posted a farewell message on X on his last day, thanking the Apple community for 15 years of inspiration, and Apple's Instagram shared a "Thank you, Tim" tribute. He also sent a final memo to employees saying he'll miss the work but is at peace with his decision. John Ternus, Apple's longtime senior vice president of hardware engineering, takes over as CEO starting today. Ternus is a mechanical engineer who joined Apple in 2001 after graduating from the University of Pennsylvania, and has spent 25 years at the company. Key numbers: - Cook became CEO in 2011, serving 15 years - Apple's market cap grew from about $350 billion to roughly $4.5 to $5 trillion under his leadership - That's about 14x growth, or roughly $4.3 trillion in added market value Cook becomes Apple's executive chairman as Ternus steps in as the company's next chief executive.