Steve Jobs resigned as Apple Inc Chief Executive on Wednesday, without specifying a reason.

Tim Cook, the company's Chief Operating Officer, who has been standing in for Jobs during his medical leave, has been named the new CEO. Jobs becomes Chairman.

Apple's shares fell 5 percent in after-hours trading.

The following are instant reactions from analysts and investors.

JASON HIRSCHHORN, FORMER CHIEF DIGITAL OFFICER, MTV AND CO-PRESIDENT, MYSPACE

I have to imagine that it's related to his health. I cannot imagine another explanation.

If you read his note, this is clearly not a surprise to the board, they've discussed succession.

Steve Jobs is my hero, a legend in global industry and a true Willy Wonka-esque deliverer of joy and happiness through product and experiences.

But you need to believe that he didn't do it all on his own and that Tim Cook is the real deal. Apple has a very, very deep bench of talent and the product roadmap is stellar and their ability to execute and supply it is beyond compare.

ANONYMOUS SILICON VALLEY CEO

It's really sad. No one is looking at this as a business thing, but as a human thing. No one thinks that Steve is just stepping aside because he just doesn't want to be CEO of Apple anymore. It feels like another shoe is going to drop.

HENDI SUSANTO, ANALYST, GABELLI & CO

(Jobs) has been elected as the chairman of the board. The questions about his health may come back even in his position as a chairman.

Apple will need to show that without Jobs it can come up with visionary products. People view Cook's leadership as more on the executive and operations side and not as a visionary.

I had a 'buy' recommendation on Apple and I believe that Apple can still innovate. Apple can remain a strong player in innovation.

The timing is quite strategic, considering Apple is expected to announce the new iPhone soon.

MICHAEL WALKER, PORTFOLIO MANAGER, WP STEWART

Frankly it removes an overhang. People didn't know when this would happen or when the day would come, but now we know. It's probably the best possible outcome.

I don't expect a big sell-off from this. There's a shock there because it's the middle of August. A lot of people are out and its thinly traded so you do get loud moves.

I'm not saying it's a good thing that Steve Jobs is not CEO -- he's a genius -- but he's not really going away. He's staying on as chairman. I would be much more discouraged if he wasn't going to be involved at all anymore.

I don't know if anyone knows how his health is. People thought he'd be gone long ago and he's outlived all those alarmists. He's got a tough condition and that's understood.

SCOTT SUTHERLAND, ANALYST, WEDBUSH SECURITIES

First of all, the product pipeline is pretty much set for the next couple of years.

The battle is moving to the cloud and connective services. It's a chance to hire some people with those capabilities. That's what's going to be more important in the future.

They are in capable hands for the next couple of years. Any weakness I'd tell investors to buy.

DANIEL ERNST, ANALYST, HUDSON SQUARE RESEARCH

I certainly wasn't expecting it this evening but this is his third medical leave, this one has lasted roughly eight months, so I don't think it's a terrible shock either.

The foregone conclusion was that we wouldn't have Steve as CEO forever and there was just a lot of speculation about what the succession plan was and now they've made it very clear -- it's Cook.

He (Cook) has done in the COO role a tremendous job in running this company and bringing new products to bear. Should the stock trade down on this? It's going to be a great opportunity because this is a growth company.

In each of the three medical leaves that Steve has been on they have released new products. The rest of technology is stumbling and this company is growing at dramatic rates.

The quality of a good leader is being able to put together a team around you. Steve doesn't design every aspect of these products the way the mythology says he does. There are guys who run the floor sales at Apple retailers that could go out and be head marketers at this company. They've done a great job of imprinting Steve's DNA across the organization and they will continue to innovate and take share.

KEVIN DEDE, ANALYST, BRIGANTINE ADVISORS

The real takeaway is not to underestimate the bench. He's got an amazing cast supporting the operation of the company.

The acceleration you've seen in product introduction over the past five years is a real tribute to the team ... the talent that he's been able to attract is absolutely phenomenal.

The big concern is 'Uh-oh, is this the end?' I think that would be clearly overblown.

He's an extraordinary individual with incredible vision and drive. I'm sure he will be sorely missed, but I don't think the company spirals out of control. It's not like this is surprising news.

ANONYMOUS LONG-TIME APPLE SHAREHOLDER, SOUTHEAST U.S.

In the pantheon of American business there may be another person who should be elevated to the business Hall of Fame for two lives. But he's the only one who should be in there for three lives. This is for his first incarnation at Apple, then Pixar and then the latest Apple period.

Apple will be a very strong financial company for decades to come. But it's not on autopilot. They will ultimately need someone who is at minimum a strong decider. To remain at the top they'll need a new visionary.

MICHAEL GARTENBERG, RESEARCH DIRECTOR, GARTNER INC

While this marks the end of an era for Apple, it's important to remember the there's more to Apple than any one person, even Steve Jobs.

Continuing as chairman Mr. Jobs will continue to leave his mark on both the company and products even as he transfers the reigns to Mr. Cook.

ASHOK KUMAR, ANALYST, RODMAN & RENSHAW LLC

Given that it's just ahead of the iPhone 5 release it is a surprise because this company is near and dear to his heart and he has been instrumental in re-architecting this turnaround and its explosive performance. So the fact that he is stepping down ahead of a much anticipated product release is not a good sign.

(Tim Cook) is very highly regarded internally at Apple. From a succession perspective they could not possibly identify a better candidate. He has a track record to back it up. What Steve Jobs leaves behind is a very deep and a broad bench and so they will continue in his footsteps.

Steve Jobs is not your typical CEO. He is both a visionary from a technology industry perspective as well as a micro-manager. To say that he is instrumental or key in the turnaround of Apple's fortunes would be a gross understatement.

It really does create uncertainty even though the execution may not falter given the company's deep product pipeline. But the headline risk is still there.

SHANNON CROSS, ANALYST, CROSS RESEARCH

It's not a big surprise. Investors had expected it.

Investors are very comfortable with Tim Cook even though Jobs has been a driver of innovation and clearly an Apple success. Tim has shown Apple can still outperform extremely well when he's been acting as CEO during the prior medical leave.

I don't know if it's a health issue. I don't know if it is a shock. Most likely it was going to happen at some point. Why today versus another day? I don't know.

It won't affect next the iPad or next iPhone. Apple's product line-up is well set. Steve, keep in mind, has been training people at Apple and there is a culture at Apple that is very strong.

FRED WILSON, VENTURE CAPITALIST AND PRINCIPAL, UNION SQUARE VENTURES

I'm sad. He's the greatest entrepreneur ever.

(Asked if Apple's future is secure without Jobs)

At least in the near term.

COLIN GILLIS, ANALYST, BGC FINANCIAL

I will say to investors don't panic and remain calm, it's the right thing to do. Steve will be chairman and Cook is CEO.

This is not a surprise or unexpected. Steve is going to be able to provide the input he would do as a CEO. But Tim has been de facto CEO for some time and the company has been hugely successful. The vision and the roadmap is intact.

JEFFREY FIDACARO, SUSQUEHANNA FINANCIAL GROUP

It's a prudent move to name a successor that is Tim Cook.

I don't think the Street will find that unexpected. It's nice to get a succession plan in place with Steve Jobs still at the helm of the board and guiding Tim Cook.

(Reporting by Liana Baker and Yinka Adegoke in New York, Alistair Barr in San Francisco, Nichola Groom and Lisa Richwine in Los Angeles)

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