Unlike most other travel sites, which generally want you to book right away, Hopper is predicated on patience. Aimed primarily at the leisure-travel market, the app uses a sophisticated algorithm to predict the best deals and times to fly and buy–alerting users at the optimal moment, well before prices go up or down by specific amounts.

Since its launch in 2015, Hopper has become the most formidable newcomer in an $800 billion flight market dominated by Expedia and Booking Holdings(previously called Priceline), and now boasts more than 20 million users.

In January, Hopper was the fourth-most-downloaded travel app in the U.S., after Uber, Lyft and Airbnb. The 120-person company, led by Lalonde, 44, and cofounder and CTO Joost Ouwerkerk, 46, generated about $15 million in revenue last year by selling flights, almost entirely through push notifications.