Apple is entering a major transition after CEO Tim Cook announced he's stepping down,

following 15 years at the helm of one of the world's most influential tech companies.

Cook will hand over leadership to John Turnus, the company's head of hardware engineering.

But the handoff comes at a challenging moment for the iPhone and computer giant as it confronts a series of big questions about its future.

William Brangham has more.

During Cook's time at Apple, the company's market value surged by more than $3.6 trillion, that's trillion with a T, dollars, thanks in large part to the dominance of the now ubiquitous iPhone.

But starting September 1st, John Ternus will step into the top job.

He has spent 25 years leading Apple's engineering team.

That's the one that built products like the iPhone, the iPad, and Mac computers.

So for more on what we can expect from Apple,

in the coming years, I'm joined by Rolf Winkler.

He's a technology reporter at the Wall Street Journal

who has covered Apple closely.

Rolf, thank you so much for being here.

Before we get to the new boss,

let's talk a little bit about Tim Cook's legacy. I mean, he took this job

arguably handling what was probably the worst, the hardest succession gig in the tech industry

following in the footsteps of the revered visionary Steve Jobs. How did Tim Cook do? He did,

well, it depends on who you ask. If you ask shareholders, he did pretty well, you guys

mentioned the number, $3.6 trillion of value added during his tenure, that's pretty

impressive. If you ask others who are looking for innovation, really truly new products

from Apple, they haven't shown us anything, you know, certainly as game-changing

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