Booking.com is the biggest travel company in the world focused on accommodation - what CEO and president Gillian Tans calls “the most crucial part of the trip.” But in recent years, the more than 20-year-old brand - the largest part of Booking Holdings, which also owns the likes of Priceline, Kayak, and OpenTable - has started to feel some serious heat from U.S. upstart Airbnb. In April, Booking.com said that it had hit 5 million non-hotel listings, a figure that had grown 27% in the last year. Airbnb, meanwhile, is hot on its heels with 4.85 million listings. Get the full story at Fortune View also "How disruption keeps booking.com one step ahead" at Fortune (Video)