After your startup’s first sales, the next phase is growth. Congratulations—you will now enter one of the trickiest phases of your entrepreneurial journey.

Why? Scaling requires a massive transition, where at any point one or several things can go wrong. The simple reason why so many startups fail during this phase, despite a great idea and proven product/market fit with early customers, is execution.

Execution can relate to marketing, operations, technology, logistics, software, systems, etc. It’s both any aspect of business building and your capacity to deliver what you promised, on time, and within certain financial limits independent from external factors.

Execution in many ways becomes more important than your idea or minimum viable product (MVP). To execute well, you must get stronger, not just bigger. You must learn what you’re good at. You must balance the tension with your customers between scale and intimacy. Finally, you must use the hurdles you surmount as a competitive advantage to grow fast and well.

For our book, Smart Startups, we interviewed 18 entrepreneurs who graduated from Harvard Business School, probing them about what they discovered along the way and what they wish they had known before launching their businesses. Based on those conversations, here are the two most important keys to executing well during a startup’s growth phase.

1. MANAGE THE GRIND AND CHAOS

Once you’ve gotten on the growth curve and have figured out a solid go-to-market plan to scale the business, you need to deliver. That is when many entrepreneurs realize that their model is not yet scalable. Many insufficiently anticipate what is required in technology, operations, supply chain, and back office to make growth happen in a sustainable way.

It can be a long, intense grind to iron out the kinks as you scale. Matt Salzberg from Blue Apron explains the grind well. Blue Apron grew its customer base fast, thanks to its innovative customer loyalty program. Once the growth kicked in, the company lacked some of the required systems, which impaired its execution.

On the operations front, it was a constant struggle to make the supply chain come together to ship a box every week. And so we’d have periods of time where we wouldn’t ship all of our deliveries on time. We’d have to tell customers, “Sorry, you’re not getting your box this week because we had an upstream supply chain issue,” or, “Our labor scheduling and management in our fulfillment centers wasn’t done appropriately and we didn’t have the manpower to physically do all the work we had to do.” There were constantly issues with trucks breaking down.

I would describe a lot of the problems as upstream supply chain planning because we had not yet developed all those systems. And we were managing the complexity without robust systems to manage labor planning in our call centers, upstream supply, purchasing, and more. A lot of it was done manually through intuition and spreadsheets.

I also wish we had invested in more robust HR systems and processes earlier on in the company’s life in order to be more prepared for that huge growth in employee population.

Matt Salzberg, Blue Apron

As Matt explains, many of the problems could have been avoided if they had invested earlier in the right systems. But it is not always easy to anticipate and know what to invest in. The problem is that operations might seem manageable and simple in the early days, but, as you grow, the sheer volumes create the complexity. Josh Hix from Plated explains how increasing volumes created a true challenge.

The hardest thing is the sum of the parts. It’s not one thing. It’s not that hard to package ingredients. It’s not that hard to write a recipe. It’s not that hard to acquire a customer. The challenge is the operational intensity and the cadence. And it’s gotten bigger and more complex over the years.

We write a set of recipes and then do some fairly sophisticated demand forecasting to manage food waste. And then receive, and then portion, and then kit, and then ship, and then deliver, and then do the customer service for said recipes every single week. It’s that cycle and the speed of it and the turnover that creates the difficulty, in my opinion. It’s doing all of that and doing it over and over at Six Sigma levels of quality and accuracy. It’s not one thing. Retail is detail. What is hard about what Walmart does? Buying. Stocking the shelf. Selling. It’s none of those things; it’s doing everything right every single day in a way that is high quality so you can drive low prices.

Josh Hix, Plated

There are a couple of things to keep in mind about executing at scale.