The top executives at Uber and Airbnb share top honors in Fortune magazine’s latest list of “40 Under 40.”

They’re revolutionizing two very different industries, but this year was the year that Kalanick and Chesky became the figureheads for the ongoing clash between the great potential of the Internet economy and the rules and regulations of the old economy. In the Internet economy, the web can connect hundreds of millions of us to powerful platforms and networks where we can help ourselves to extra inventory, whether it’s a spare bedroom in someone’s house or a driver-for-hire idling in between jobs.

In the real world economy, that ability can collide with entrenched interest groups and decades-old regulations. Kalanick’s style is to enter a market, cease-and-desist orders be damned, and ask questions later; Chesky’s is more diplomatic. What they have in common is growth. In the last 12 months, Uber has quadrupled the number of markets it serves to over 170 cities (you can now Uber in Abu Dhabi and Hyderabad); This June, it raised $1.2 billion at an $17 billion valuation, one of the highest ever for a tech startup. Airbnb has provided shelter for 20 million people—half of those this year—and on its peak night this summer, 425,000 people stayed in an Airbnb somewhere in the world.