When you first even think of building a company from scratch, a wealth of emotions washes over you: excitement, mostly, but also probably a little bit of nervousness. I would know — I left Salesforce in 2007 to start BrightEdge. But after taking the initial plunge, I found myself alone in my pursuit of BrightEdge and thought, “So this is what startup life is all about?” It suddenly became clear: I traded a stable career to try and transform the marketing industry without a business partner by my side. And that just wasn’t going to work.

Finding someone to start a business is challenging yet essential. This person has the potential to make or break your business. For me, it came down to finding someone that matched my level of enthusiasm, but possessed an entirely different set of skills. Here’s how I took on the task:

Passion comes first
It may sound obvious, but passion is the single most important element of selecting a business partner. Today, most employees only stay at a company for about four years — so you need someone whose enthusiasm will keep them engaged for the long haul. While excitement is key, it’s that deep-rooted passion for changing, fixing or eliminating a problem that indicates someone is worth your investment.

See also: How to avoid picking the wrong business partner

When I was searching for a co-founder before starting BrightEdge, I met Lemuel Park through friends. He was at a crossroads in his career, and had always wanted to start a company. So when I found out that he’d recently spent an entire year in West Africa helping the United States Agency for International Development (USAID) build their infrastructure (with no running water, by the way) I knew he was an “all in” kind of guy who lets his passions guide him.

Don’t forget about hard skills
While Lem’s commitment to BrightEdge’s mission was the first thing I looked for, I couldn’t help but notice his stellar background and how much it complemented my own. During his time at Ernst and Young, Lem was tasked with figuring out security vulnerabilities by breaking into banks. In other words, he was a tech wizard — exactly what I needed to help me build the infrastructure of the BrightEdge platform.

See also: 3 signs you need to ditch your business partner

I’m sure you’ve heard this before, but I’ll repeat it because it’s so crucial: the best partnerships are those that complement each other. Find someone with the skills you don’t have, and you’ll make each other stronger in the process.

Make sure they can weather the storm
Building a business from scratch is not for the faint of heart. It takes hard work, a whole lot of sacrifice and a business partner who’s willing to experience the ups and downs with you. When Lem and I were starting BrightEdge, we both sacrificed a lot to make BrightEdge actually succeed. I lived with my wife and newborn daughter in a 1,000 square foot apartment — and Lem moved in with us to cut costs and maximize productivity. With Lem on the couch and half of the kitchen table covered with servers helping us index the web, we started our company and never looked back.

Read all responses to the Leadership Insider question: What do you look for in the ideal business partner?

The most important relationship you will make in your career by Nirav Tolia, CEO of Nextdoor.