Such early success requires a lot of things: intelligence, vision, persistence, an appetite for adventure and risk. What it doesn’t require is a clean, smooth, upward trajectory.

Like all of us, these business leaders have had their share of terrible days, weeks, and years. They’re human, after all.

Fortune asked this year’s 40 Under 40 to talk about some of their lowest moments. Below are some of their responses.

Jon Stein, CEO, Betterment (No. 27)

Stein is confident about Betterment’s future. And for good reason: The automated investment service has $6 billion in assets under management for its 175,000 customers, and recently launched a service to help companies manage employees’ 401(k) plans.

But that wasn’t always the case. In the beginning, when the service was more idea less company, there was the stress of “walking away from a cushy consulting job and post-business school job offers,” a decision he “didn’t take lightly.” In addition, he worried the professional demands of launching the company would mean he couldn’t give his kids the same “level of engagement my parents gave me.”

But the lowest came after he had committed to the company:

When our CTO left, and we still didn’t have funding, before we’d launched, I wasn’t sure we’d make it. That was a dark day.

Joe Zadeh, Airbnb (No. 22)

In 2010, Zadeh, 35, joined a home-sharing company as its ninth employee. Since then, he’s watched the company morph from a little-known company to a household name with more than 1.5 million in 191 countries, and a reported valuation of $30 billion.

Related: How an ‘Insufferably Arrogant’ Kid Transformed Into a 38-Year-Old CEO

Airbnb’s explosive growth has been exhilarating. But it’s also meant constant, sometimes troublingly unhealthy, pressure:

A few years ago there were a ton of things going on at Airbnb, we were growing much faster than expected, and I felt as though I was in over my head. I wanted to control it all, so I worked myself into the ground. I neglected to take care of myself and ended up getting pneumonia. That forced me to stop work completely for a couple of weeks. I think that was a valuable lesson in the obvious: I have real limits. When I look back I realize I could have trusted my team more to help. Not only would they have been willing, they would have loved the opportunity. It’s unfortunate I had to jeopardize my health to learn that lesson.

Ryan Smith, CEO, Qualtrics (No. 12)

Today, Smith is the CEO of a thriving, profitable business that is expected to go public in the next couple years. But back when he first launched the research software company, the future was far from certain, particularly because Smith felt the company was lagging behind competitors on a variety of fronts.

It wasn’t the most relaxing of times:

Very early on, I felt like we were way behind. Everyone in our space had raised significant amounts of money and were hiring strong teams. We were a two person company. Our website didn’t look good. We had no money. I went to my dad and started asking a lot of questions. “Why don’t we do this? How come we haven’t done that?” He just looked at me and said, “What’s stopping you from doing it?” It took me a few days, but I realized then that if things were going to happen, it was up to me make them happen. Once I realized he was right, it was game on.

Adam Grant, Professor of Psychology, Wharton School, University of Pennsylvania (No. 28)

As a best-selling author, well-known management consultant (he recently completed a stint at Google), and friend of Sheryl Sandberg’s, Grant is one of the most well-known university professors around. But that doesn’t mean his career path got off to a seamless start. In the beginning, the negative feedback far outweighed the positive.

His lowest point?

Having my articles rejected over and over—and then after my first job interview, being told I would never make it as a professor.

Katherine Power, CEO, Clique Media Group (No. 35)

Power, 36, founded Clique Media Group, a media and commerce company that focuses on women’s brands, in 2006. It’s been a steep learning curve ever since. “I am very shy by nature and a bit socially awkward, so I’ve definitely had to work really hard on quelling that anxiety because there is no room for that as the CEO,” Power says. “My office door is a revolving one, and I’m constantly in meetings with people I don’t know, so I’ve had to work on getting more comfortable in social situations.”

Social anxiety aside, one of her lowest moments led to an epiphany that would shape the rest of her career:

My lowest moment was also the best thing that ever happened to my career. In 2005, I was the West Coast editor of Elle and Elle Girl magazine. Jack Kliger (CEO of Hachette Filipacchi Media, owner of the two titles at the time) called the entire Elle Girl team to a conference call one afternoon. It was then that he announced they would be folding Elle Girl magazine and turning it into a website because “young people aren’t reading magazines.” That sparked an idea. I was in my mid-20s, spending all my time on the computer, but at that point Elle.com was just a splash page to collect print subscription orders. There was nowhere to consume quality magazine content on the internet. From there, our first website, WhoWhatWear.com, was born.

Hooi Ling Tan, Co-Founder of Grab (No. 17)

As co-founder of the ridesharing company Grab, which operates in Southeast Asia and is reportedly valued at more than $3 billion, Ling Tan is something of a public figure. The role wasn’t an easy fit, at least at first. “I am an introvert by nature. Over the years, I’ve learned to be more vocal and proactive in discussions,” she says. “ I’ve also learned to put myself out there for public speaking.”

Luckily, she has experience recasting difficult situations as learning opportunities. One of the lowest moments of her career also taught her the value of stepping outside her comfort zone:

My first three months in McKinsey, I felt like a fish out of water. Having trained as a mechanical engineer, I knew nothing about what it takes to be a consultant. I was also working with some of the smartest people in the world. It was tough but extremely rewarding.

Paul Judge, Co-founder and CEO, Co-founder and Executive Chairman, Luma, Pindrop Security (No. 33)

The Atlanta-based entrepreneur founded PureWire, an Internet cybersecurity company acquired by Barracuda Networks in 2009. Shortly after that, he co-founded Pindrop Security, which detects and blocks phone fraud, and now serves as chief executive for Wi-Fi device maker Luma.

Related: Lin-Manuel Miranda Thought ‘Hamilton’ Would Only Appeal to History Teachers

Managing people took some getting used to. “I’ve had to work on my natural tendency to be introverted,” he says. “This means there is effort that goes into social engagement such as small talk.”

He’s also learned that being a leader can nearly break your heart:

In 2008, our company of 40 employees ran out of money. We had forty families depending on us for their jobs and paychecks. We found ourselves in this position because the financial crisis of 2008 unfortunately lined up perfectly with when we were scheduled to do our next round of venture funding. Investors that were committed suddenly could not write checks. Many people advised us to let go of the entire team and shut down. Instead my founder and I put all of our personal savings into making payroll for months to keep the company alive, keeping food on the table for those 40 families. Several months later we had a successful acquisition. Our investors made money and all of our employees kept their jobs. Those months were the lowest moment of my career.