Tim Cook marked his final day as Apple’s chief executive with a letter thanking staff and reflecting on his 15-year leadership. He said Apple’s success stemmed from its culture, shared purpose and employees, while expressing confidence in his successor, John Ternus; the transition matters because Cook led Apple through a period of major global growth and influence.

Cook became CEO in 2011 after Steve Jobs and oversaw Apple’s rise into one of the world’s most dominant technology companies. In the message, he stressed that he is not leaving Apple, but will move into a new role and continue to be seen at Apple Park and internationally; he described Ternus as capable of leading the company’s product development.